I think that I've answered this over in 'Law' but just to clarify Shadow Man's post:
If you attempt to keep the money, you'll be committing a criminal offence. However, as long as you make it clear that you intend to pay the money back eventually, it's only a civil matter. Your employer could still pursue you through the courts but a court wouldn't order you to pay back the money at a rate greater than that you could afford. Write a letter to your former employer, as follows:
"Dear Mr Bloggs,
I acknowledge receipt of your letter regarding overpayment of salary to me during the period of my maternity leave.
I further acknowledge that I must repay this money and it is my intention to do so.
However, I wish to advise you that I am currently unemployed and my only income is derived from state benefits. Further, I have no savings. I can only offer to make repayment at the rate of �1 per week for the foreseeable future but I will, of course, make an improved offer when my finances are better.
Yours faithfully,
B. Bee ".
Such a letter will ensure that you can't be prosecuted under the Theft Act. Further, as it's unlikely that the employer could be bothered with processing weekly payments of �1, you'll probably simply receive a letter telling you that your offer is rejected but the employer will contact you at a later date to see if you're able to make an improved offer.
Don't worry too much about it. You are legally obliged to repay the money but your former employer, and the civil courts, have to accept that you can't pay out money you haven't got.
Chris