Completely and utterly gutted

If you are being accurate about your calorie counting (cups of tea etc not withstanding) then you have not gained 6 lb of fat.

If you are following MFP as designed, then you are presumably eating at a calorie deficit of 500 (to lose 1 lb a week) or up to 1,000 (to lose 2 lb a week). To gain fat, you would have to not only eat those extra calories to eliminate your calorie deficit, but you'd have to eat an additional approximately 3,500 calories per lb of fat. You haven't done that.

Weight loss is not linear. Really. It goes up and down all the time, even for people who are maintaining. It fluctuates constantly due mostly to the amount of water in your body at any one time. There are lots of things that can make you retain water, including an increase in sodium intake, an increase in carbs, a new exercise regime, or an increase in exercise intensity and hormonal fluctuations. Women often gain pounds right before their period, so that might be an issue. If your muscles are sore, then that's an indication that they are retaining water as they repair themselves - completely normal. If you track your sodium and notice a recent increase in that - that could explain the fluctuation too. Another thing to be aware of is that you will weigh differently at different times of the day, so make sure you are consistent with the timing, scales, and what you are wearing.

I've weighed every day since January, and although I've lost 43 lbs in that time, the number on the scale has gone up and down all the time. It probably goes up about 3 days a week on average. What matters though is that the overall trend is downwards.

I know it's frustrating, and to see that sudden gain for the first time must have been a shock. Honestly though, you can't let it get you down, because it will happen sometimes. Weight loss is not easy, and it takes time. Looking at your profile, it doesn't look like you have much to lose (unless you're aiming for a mini goal). When you are close to a healthy weight, it takes even longer, so it can be frustrating. Just keep doing what you're doing, and you will see results. You've made some really good changes to your lifestyle already, so you should be really proud of that. If the scale is getting you down that much, it might be worth stepping away from it and only weighing yourself every couple of weeks. You can use measurements and photos to see progress in other ways. 0.5" off every measurement is a massive achievement in a week! The scale is a useful tool to measure progress, but it doesn't show the whole picture, so try not to get so emotionally attached to that number.

I think it's an ok gain if it's water and not fat, and based on what you've said, it's not fat. If you are retaining water, drinking a lot of water will help to get rid of it, but otherwise just keep at it. You can lose weight without doing any exercise at all - just be creating a calorie deficit with your diet. From the sounds of though, being close to your goal weight, you'd benefit from the kinds of body composition changes you get through exercise. Strength training can make a huge difference to the appearance of your body - losing inches and getting visible muscle tone. If you don't have the option of going to the gym or using weights at home, then 30 Day Shred and other exercises using your bodyweight can be a good start. It's not easy fitting it in when you've got other obligations, but find whatever works for you - like doing it in the morning if you can.

Here is a little tough love.

Two weeks is not enough time to make any decision on how your program is doing. Expecting amazing results in a really short time is not realistic. You have made some MAJOR changes to your lifestyle and your body is going to need time to adjust. It also sounds like you dont have that much to lose but have put yourself in a very large deficit. This will only add to the shock on your body and may not provide the nutrients and energy needs your body requires. There is a reason -1lbs per week is recommended. Why so many people choose to ignore that baffles me. Personally I can fluctuate 5 pounds in one day so 6 lbs in a week is not that bad, especially after what you forced your body to adapt to.

My advice is to adjust your calories for a 1 pound per week deficit and then give it 3 months to decide if there needs to be any changes made. Fix your expectations and this will work out for you. Expect unrealistic results and you will only frustrate yourself.

I'm not trying to lose more than 1 lb a week. When I started I had 7 weeks till I see hubby, and thought that at a rate of 1lb a week, that would be perfect, i'd have lost half a stone when he comes home for his R&R. I was pretty gobsmacked when I lost 3 last week. But the I was also pretty gobsmacked when I weighed this week and was heavier than when I started!

Will definitely keep going, and am going to up my water intake as I don't think I am drinking enough (probably about 2 pints a day at the mo).

I'm not sure how to adjust my calories. I don't feel hungry. In fact I sometimes struggle to meet my 1200 if i've had a particularly healthy day! Might have to start eating biscuits in the evening, (but surely that is bad cos of the fat in them?) Blimey this is all so confusing! lol

Try adding a banana to your diet or peanutbutter on a slice of toast... Helps me to increase my calories when im struggling