What cynic-al and peterfile have said, several times over. Speak to a lawyer. Get good advice early. And you don;t want a family lawyer – you need someone who understands commercial disputes. The “family” part of this is misleading.
Two things that leap to mind:
1. If you haven’t breached the terms of the “loan”, then suing you to repay it can’t work. If I borrow £34k off the bank with a repayment term of 10 years, they can’t decide at year 6 to sue me to repay the balance. I haven’t done anything wrong – the court wouldn’t wear that sort of nonsense.
2. If this is the first contact you’ve had about the money, then I suspect his lawyers know full well they can’t just go ahead an issue proceedings without getting a kicking from the court for failing to follow the Civil Procedure Rules.