Thanks everyone for your help. To answer some questions, the weight gain was caused by an increased, insatiable appetite and associated changes in my eating habits although I remained active. Trying to lose weight, I was eating about 1000 calories a day (so the starvation mode thing could be right) until I realised that my calorie count was that low. I've increased it to 1200, which is the recommended amount for women trying to lose weight, although my RMR is something around 1800 calories a day, so at least theoretically there should be lots of room to manoeuvre?

Is it possible to be on 'starvation mode' when my diet, although small, is nutritionally sound?