Thanks, and where do i stand with paying it back in installments?

Good evening, I have been trying to find out what to do with regards to my problem. Back in 2012 i was involved in a car accident. It wasn't until last year that I decided to make a claim for personal injury on the say so of a doctor(Who did not get paid by the anyone i must add!). The firm of solicitors i used were Silverbeck Rymer. To say they were useless is an understatement. I had to do all the chasing, they never rang me, when they had settlement figures they didn't bother to contact me. Eventually I got a payout and they sent me a cheque. 2 Weeks later i got another cheque for the same amount. Knowing they may ask for it back I put it in the bank, half expecting them to spot the mistake and cancel the cheque. Now I know I shouldn't have cashed it and told them of their mistake but I felt like I had one over on them for once! They have wrote to me now, nearly 5 weeks after the second cheque had cleared and basically said we want our money. Seeing as they have overpaid me, both cheques had 2 separate signatures on and it was clearly their mistake do I HAVE to pay it back? I suspect I do. If I do, where do I stand if I offered them a % of the overpayment and the rest monthly? Can they force me to pay it back in 1 lump sum? Oh and while I remember, no i would no recommend them to ANYONE!