You should have done some research into the average salary and remuneration that this type of position will pay. Try to deflect the question by turning it around and asking the interviewer about the salary on offer. Typically, they will start with a lower figure than they are prepared to offer because they want to keep their costs down. So if you are pressed to give a number, its best to give a range to avoid pricing yourself out of contention.

For example:

I'm sure whatever salary you're paying is consistent with the rest of the market average of £23,000 to £25,000.