Thanks for reply, :) I think there are a couple of things - firstly improvements to snacks etc, and then also some 'psychological' tricks you can try and use to cut down on snacks/make better choices.
Firstly, I must declare that I am no expert but I am very in to nutrition and exercise and have a health sciences degree and I find this kind of thing very interesting and also like to try and help people. I just felt I should say before I go dishing out advice :).
I think with the walking, as much as possible but obviously without overdoing it if you're not that fit. I would try and do 30 mins every day, but have you thought about 30 mins (or maybe less time if you thought 30 mins was too much) twice a day? Could you go out in the morning, and then in the afternoon? Doing this twice a day would get your metabolism fired up twice. Then think about what other activities you do in your daily life - it can all add up - housework, gardening, cleaning the car etc - if you do these things already, try doing them with a little more vigor, and it will all contribute to improving your stamina etc, so that you can build up to other forms of exercise. Without knowing your fitness level, you could also do some kind of challenge (I have done this before) where for 5 mins in every hour (set an alarm) you will do some form of exercise - squats, lunges, dancing around to your favourite CD etc - whatever floats your boat!
Your diet, as you say is not drastically unhealthy, but is very typical of our British 'spiky blood sugar' diet. Can I suggest that the first meal you try and overhaul is your breakfast - weetabix and banana is 'healthy' in that it contains nutrients and is low fat, but as it's mostly carbohydrates it will spike your blood sugar which will then drop after a couple of hours, and even though you're not hungry, this is why you reach for the biscuit tin. Do you like eggs? Eggs on wholemeal is a perfect breakfast, the protein will help stabalise your blood sugar, and also, I find that by having a savoury breakfast, you don't 'kick start' your sweet tooth so early in the day either (something I have had a big problem with).
I used to have a massive biscuit habit, and what helped me give them up was when I realised that 4-5 biscuits a day (which is fairly easy to do) is (depending on the biscuit) approximately 250 calories which over a week alone is almost a whole days worth of calories for a woman! Like you I also enjoy meeting friends for cake - I decided in the end that as I like cakes more than biscuits - much more delish, that I would give up all 'boring' shop bought biscuits and snacks so that when I go out with friends I could have a piece of cake without worrying too much. As for snacks now, I find the usual chopped carrot/celery/apple/peppers pretty good - if you cut down on the amount of sugar you consume, then things like this will actually taste really naturally sweet on their own. Also, eat a few nuts alongside (although watch portion control - a few means no more than 6 brazils, or 10 other nuts which are smaller, but eating nuts or seeds with fruit/veg will keep you fuller for longer.
Again, on the cakes with friends - realistically if you want to lose weight, try and restrict yourself to no more than 2 a week - a big slab of cake will probably be 600 calories - so bearing in mind a 2000 calorie per day amount for weight maintenance, this is a big chunk of your daily amount - but if you've been really good all week, then you should make sure you have something delish as a treat. Does the coffee shop you go to sell smaller things that you could have (like a small biscottti/shortbread etc) some of the times so you don't feel like you're missing out?
My only other advice is if you are drinking any sugary drinks - so fruit juice or coke etc, cut it right out and have water instead - again, you can take in a suprising amount of calories just from one glass of OJ.
Best of luck - feel free to keep in touch on this thread :)