You might want to read other posts of mine where I've advised young guys on how to gain muscle mass. In short: sufficient protein intake while stressing the muscles along with rest days, e.g. push ups, curls with wt's., running and biking for building legs. Consistency is the key.
BTW, many people would envy you for your lean-ness.

I would have to disagree with the calistenics/cardiovascular training. That would promote toning and the loss of fat, I think the user wants to bulk up, even if it means the fat he may currently have is still around. But people have achieved the goal in different ways.

I personally quit sports in order to gain weight. I don't know what 11 stone is off the top of my head, but I'll assume you have an ectomorphic body type like myself. If you don't know what that means, it is just a terminology used to describe 1 of 3 main body types. Ectomorphs are classified as(but is not limited completely to all of the following) tall height, lean body build, weak skeletal structure/frame, double jointed or hyperflexibility, short upper torso, lanky, high metabolism.

The other two are Mesomorphic and Endomorphic, which isn't your main concern (assuming you are ectomorphic), but generally speaking mesomorphs are the perfect body building type, and endomorphs are short and have low metabolism.

These are genetic factors, so with few exceptions, we cannot change it. Considering this, it is probably apparent to you that there has to be some sort of variance between the way each of these body types have to exercise to achieve the same goals. For instance, endomorphs have a very low metabolism and have to exercise in more repetitions and more cardiovascular to burn calories far more then an ectomorph would (since they have a high metabolism to burn the calories anyway).

So, considering how much faster your metabolism may already be compared to the average person, would you say you need to cut down on cardiovascular activities, or increase it? You need to decrease the amount of cardiovascular exercises (i.e sports) significantly to conserve the energy you need to weight lift anaerobically.

Presses, i.e bench pressing, shoulder press, squats, and other very demanding anaerobic exercises should be your focus if you want to get bigger. There are 4 different ways to train your muscles, power training, strength training (yes the two are different), hypertrophy and lastly endurance. Strength means the amount of force that one can exert on an object. Power is the ability to exert force over a [short] period of time. Ex. Strength - being able to lift 50 pounds. Power - being able to lift 50 pounds up into the air in .2 seconds as opposed to .5 seconds. Hypertrophy means muscle growth, and endurance is the capability of your muscles to well, endure doing an activity repetitively.

Thus, your particular goal is Hypertrophy. Note however, that just because you will be exercising with Hypertrophy as your focus, that you will not increase in other aspects such as strength. You will gain other attributes, its just focusing on that particular region.

Moving on, generally speaking, in order to focus on specifically Hypertrophy, this is what you first need to do. Go to your gym, make sure you give yourself a good warm up (but not tiring yourself) so that you won't get injured for what you are about to do. Try to have a friend spot you if you are new to the weight lifting world. Step 1 is to find out what is your max rep, meaning, the most amount of weight that you can lift in that particular exercise, 1 time (with good form and not breaking yourself). In order for it to be considered your max rep, you should barely be able to do it a second time, or not be able to do it a second time. It is probably best that you use lower weight then what you personally think is your max rep and estimate (because attempting this is very accident prone). After you figured out your max rep for all the exercises (some of which I will list for you to do), you will have to use somewhere between 70-80% of your max rep as your exercise weight. meaning, if your max rep for bench press is 100 pounds, you need to use around 70 to 80 to exercise. Another way, and probably easier, is that you find the weight that is right for you in such that, you can only do between 6-10 repetitions. Remember, you must use weight that will make you drop and can't do anymore between 6-10. Doing this will focus moreso on hypertrophy (lowering the amount of reps but increasing the weight will cause you to focus more on strength, and then power, increasing the amount of reps, but lowering the weight will focus on endurance).

Again, keep in mind that you have to pick the weight to be able to do 6-10 reps. (about 4 sets per exercise as well). You should be going to the gym anywhere from 4-6 times a week. Rest when you need too, don't be dumb about it, ectomorphs have a slower repair rate then others, and the last thing you need is to be forced to quit working out because you tore your ligament. Remember, cardiovascular exercises and sports should be done to a minimum, weight lifting should be your primary source of exercise from now on.

Diet is very important, but let me tell you this, protein shakes and other supplements work to a minimal effect, most of it is just marketing from companies who only think about exploiting customers/making profits anyway. Again, what matters, is your actual diet, and how you exercise, not some "ultra super technologically enhanced whey protein created from Dr. Mikolas, a Exercise science researcher of 108 years" drink. So sure, if you want to continue to drink it up to sustain you before and after your work outs, thats fine, but don't expect a miracle out of it.

Your diet should be spread out more then the typical "3 meal a day" thing. Try eating anywhere from 5-7 times a day, and no, it doesn't have to be something huge each and every time. Whatever it is that you are eating, it should be good enough that you are full for 3ish hours until your next minimeal. Whatever the case, make sure most of it comes from carbohydrate and protein sources. I'm not a nutritionist yet, so I can't tell you the exact details of how much of each right now. Either way, the meals you are having plus the protein shake should be more then enough to supply your carb/protein source needs. You have to watch your carb amount though, because if you eat more then what is needed to sustain yourself, and more then what your body can store, will turn to fat. So try to eat complex carb sources (look up what foods are good sources of complex carbs), and try your best to limit simple carbs such as fruits, dairy products, soda, candy, etc.

I wrote an essay, I hope some of this helped. Let me know if you have other questions

*Note: I don't know your standing with experience as a weight lifter, but do your very best to focus on form and proper use of the machines or you can and probably will hurt yourself and cause complications (especially if you are a ectomorph because of our weaker skeletal structure, we are more accident prone then those darn mesomorphs)
Note2:If you are new to weight lifting, do not expect to see any size gain for possibly up to 8 weeks, because depending on your degree of exercise inactivity over the years, your neuropathways have to activate itself after the disuse. During this period, you will experience strength gain, but not size.

I wrote this awhile back.

http://ehealthforum.com/health/tips-for-ga ining-weight-t141714.html

It gives a very general outline of good foods to eat while gaining weight. When paired with weight training, this general diet will also help gain muscle.

1 stone weight gain in two weeks is not really considered a healthy weight gain. Makes me wonder just what the heck was in the protein bar.