Have you gone to see your surgeon about possible complications from the surgery? Perhaps you have an infection?

Went to see my GP. It appears I had an allergic reaction to the hypromellulose (i think that how it's spelt) in the artificial tears that I was prescribed. I have been given preservative-free tears now so we'll see how that goes.

I think you should visit your doctor, who will probably think of bacterial infection. He might prescribe you Chloramphenicol, which is one of the best antibiotic based drugs for eye infections, that causes swollen eyelids. If that drug doesn’t help than you must consider that you maybe suffer from some viral infection like conjunctivitis. They can usually recommend if they can give you some cream or if you may need something on prescription from your doctor.
If it is only one eye then it is most likely an infection and you should be able to get some antibacterial cream or ointment form the chemist. Good luck!