ragtaggle - do write it down, and you might see a pattern about what you eat, and when you lose/gain. I did the food diary thing for a while last year, and although it was pretty obvious that I never got anyway near to the 2000 calories a day which is supposed to be what most people can have there were times I would gain weight quite dramatically. Correlating what I'd eaten against that it was pretty obvious that wheat was the problem, so I've tried to cut it completely if I'm at home (not always possible of course if eating out etc) and it's made a huge difference. Try cutting things you are suspicious of out one at a time (dare I suggest Alcohol as one of them?) and you might find that there is something which is a trigger for you.
A typical day for me (and I could do with losing a lot more than a stone!) is:
Breakfast: either just black coffee or black coffee plus an apple (if I'm going to the gym first thing). At weekends I might have a small omellete with mushrooms, peppers or lean bacon, or even smoked salmon in it on one of the days.
Lunch: mixed fruit (quite a lot - about 450g of it) during the week, or sometimes soup or salad with something like ham or a small piece of cheese at weekends.
Afternoon: perhaps some grapes, or satsumas, etc
Dinner: vegetables - usually 3 or 4 different ones, plus about 200g fish or meat (probably 6 days out of 7), and perhaps (not every day) a smallish portion of something like brown/wild rice or potato and onion rosti. Will have wine with dinner at weekends, otherwise sparkling water or diet ginger beer.
That's enough, for me, to stay stable or lose - but I only need to add in one sandwich, slice of toast or portion of pasta, and I'll gain about 4lbs in a day.