Welcome Finance — repayment of overpayments

Myself and my late wife took a loan out of £2000 in 2006 over 2 years, i was informed that i had overpayed 2 months payments which added up to £315, welcome finance sent me a cheque which i put in to my bank, 7 days later the cheque was returned to me by my bank has it was not signed in accordance with the mandate. I took the cheque to my local welcome finance office and explained the problem, they said they would inform head office. Two weeks later i received another cheque which i took to my bank, 7 days later it was returned to me again by my bank. I have now phoned head office 4 times and have been told 4 times that somebody will get back to me but as yet i have had no reply. This has now gone on for 6weeks and being a single father with a 12 year old daughter £315 would help me out a little. i would be gratefull for any help with this problem.