This is a catch 22 situation I have been in myself, where it's not worth your while to accept the payrise, which is crazy.
Go and see the lone parent advisor at the job cente and ask them to crunch the numbers for you. It's about the only way to get a real result as the Government website is so vague on their figures and calculations.
I don't qualify for HB or CTB any more, and it is only just about worth accepting the payrise. This is only because my WTC & CTC calculation is based on the previous years figures, so I get an extra bit for a few months (til April), but it goes down the following year by quite a bit of the payrise, plus I actaully pay tax now (just), so I lose some of it to tax and NI too.
I always used to refuse my payrises for this reason. My firm did their best for me, for example finding me a place in the company car park which saves me £60 in parking fees every month, so this accounted in a legal way for the equivalent of the pay rises I had had to refuse.
When I discussed the payrise issue with a friend they did tell me that I should always accept the payrise, even if it made me no better off or even slightly worse off. Because the accumulated lack of increase in my salary adds up over time.
It makes me feel I'm not worth as much as other employees on some level, but it also leaves you caught in that position of always being on a low income. You won't ever get to a point where the payrise will make any real change in your income unless you start to increase your income.
Only you can decide if it's worth it. But you need the actual figures to know. So go see the lone parent advisor at the job centre. Phone up and ask for an appointment. If you do accept it you must inform the council re your HB and CT as soon as possible else you'll have a shock at the end of the year when they ask for the money back.