It all depends on your body type as to whether you can gain/loose weight easy (ectomorph, mesomorph, endomorph). I for example (ecto) can stay lean easy but cant put on weight easy - can eat cake etc etc and not exceed roughly 12% body fat.
Started training at 19, was only 9st..2 years later i was 13.5st and id guess 10% body fat (if that) so dropped fat and put on 63lbs (4.5st) of muscle (Gez you may have seen a progress pic on my Instagram?) - Now at 22 ive not progressed much more so will need to switch it up and get bigger/leaner for summer
If your trying to gain that extra 1/2 stone it should be eat, eat, eat, eat, weights, eat, sleep
Best diet for weight loss is probably Ketogenic, best for gains is just eat like made, train and supplement well
What injuries you had @
Gez ? Ive always had issued with my shoulder (rotator cuff) when going heavy on bench press or shoulder press - hurts at the time but i just get on with it now, CBA to see a doctor as they will just say "have 6 weeks break" which is a no no