Need some advice urgently!

Sorry to say but one of the key things that I've picked up is that folks 'tend' to Over estimate the amount of energy that they burn through exercise and Under estimate the amount of food that the consume ...

It might be simple maths but it's not a simple problem.

You're probably not burning a lot of energy walking around the class room all day (but for me I'd burn more stress energy having to work with kids!!!!)

And (once again, I'm really sorry) 30 mins of walking really isn't that energetic of an exercise (probably around 100 - 150 calories .. a wee snack!)

So, my advise - up your game. Accurately measure ALL food intake and start to walk further.

Best of luck with you goals. No one ever said that it would be easy (and it's not)

when using the treadmill , instead of just a steady running or walking. alternate between sprint for 30secs and walk or jog for 60sec. total of 5min to start and progressively move up to 20min.

up your calories to 1500-1600ish. nothing wrong eating at 1200 but its not good long term

try to hit these macros
carbs - 150 to 190
Protein - 100
Fat - 50

eating at 1500ish 5x/week already sets you on a deficit
then on weekends you can eat up to 1900ish when eating out with family and friends or during celebrations.

I used to have an intake of 1400 cals, would walk 30 mins per day at 5mph on the treadmill, twisting and arms swinging for maximum effect, usually being just under cals each day. This was not working, and so I figured that my body was getting used to what I was doing. So I decided to reduce cals to 1200, despite warning not to, increased exercise to 45 mins, cut out bread, and started to drink 8 glasses of water. Still seem to be gaining weight! I did have an easter alcohol binge which wont have helped. So now I need advice on how to start properly again after my binge. I am 5ft 1, medium build, pretty active (walk once per day 5mph between 30 and 45 mins), I am also a teacher that walks around the classroom all day, but my weight hovers anywhere between 11st 2 and 11st 7! I am very overweight and would like to get to 9 stone. Long way to go, but I am determined. What cal intake should I try, what exercise, anything to cut out?

1) 30 minutes of walking per day does not make you pretty active, it makes you very lightly active.
2) Swinging your arms when walking will only make you look funny, not burn more calories. Keep it up if you think it is fun, stop if you believe it helps you burn anything.
3) Walking is a great exercise and you need at least this much exercise to improve your health. But unless you plan to train for hours per day, do not rely on exercise to lose weight. Keep walking to improve your health, but focus on what you eat to lose the weight.
4) Either you are not being honest with yourself, or you are not properly weighting your food, or you are logging the wrong calories or you need to see a dr. It is not possible for someone with your weight to consistently eat 1400 calories, accurately measured every single day and not lose any weight at all, unless there some serious untreated medical issue.

don't forget that all calories are estimated more or less accurately

if you don't weight everything you consume you can be making large estimation errors - things some people forget to log a calories from fruit and drinks - yup that alcohol binge over the easter weekend

it's hard to tell as i can't see your diary whether you're measuring or guessing portions >> quick tip - measure don't guess

it's also common for treadmills and pedometers and even hrms and mfp to overestimate calories burned in excercise - they are calculating calories based on some information (usually time and/or speed and/or hrm) but the equations also make assumtions about your age, weight, byuild gender, resting heart rate ...

they are all estimations

you will know when your calories in = calories out as you will maintain a constant weight
if you are losing weight : calories in < calories out

you need to adjust to make it work for you
the mfp estimations only work if you track and log everything

I like using mfp and work it out assuming sedentary then i log excercise (lowest calorie estimation of several tools) and eat back excersise calories - i assume the bouncing round the office running errands is a bonus
you might be better setting lightly active depending on whether you do actually move aorund your classroom continuously or not

you should then set a reasonable long term weight loss target - from your profile you indicate that you're a UK 12-14 this is not nearly obese and therefore you should be targetting no more than 0.25 to 0.5 kg/week weight loss
if mfp is giving you 1200 cals it usually means that you've done your goal setting using a higher weekly loss, which doesn't work for smaller weigh loss goals like ours

i started off trying 1200 becuase i had set my goals wrong, it really didn't work, the weight loss was pitiful and i felt horrible, I'm now losing consistently either weight or inches on 1520 net.

I think what I have been doing is not eating the cals back. Also, when I log a 30 min walk at 5mph on MFP it tells me I am burning 276 cals. Is this incorrect?

The general consensus (with which I agree) is that MFP overestimates calories burned. I always eat my exercise cals back, but when MFP was the only way I had to record my exercise (I now have a BodyMedia LINK), I only recorded about 2/3 of the actual exercise that I did.

You could try the TDEE method.

Only my second post so take this with a grain of salt. What started working for me was being honest about calorie consumption. I was not logging things correctly and a bite here and there will add up and keep you from losing.

Now I log everything and I do more exercise than I log. It's the only way I can lose. I'm averaging 1.5lbs a week which I'm happy with.