I wouldn't panic just yet. I joined in the middle of last year and getting responses from recruitment was often like drawing blood from a stone! Even when I got my contract it had the helpful advise on it not to quit my old job yet 'cos of passing the medical and security. The problem was there were 14 days 'til I was due to start and my old job (like many others) needed a month's notice. Good luck anyway, maybe you got the (correct) letter this morning!