Some benefits are calculated on a £ for £ basis (so you get £312 wage rise, benefits drop by same).
Some are on tiered levels.
And some - quite often Housing and Council Tax benefits - have a definite cut off point. Go over the cut-off by even a single pound and you will lose all the benefit. Sounds like this is what has happened to you.
If that is the case, can you maybe find out what the cut-off point is and then renegotiate your wage with your employers. Might be worth letting your wages drop back down by a few pounds (technically this is probably looked on as a type of fraud ? but if the buggers force you into it?)