I had a similar issue with them last week. Flight also booked using vouchers. The reason seemed to be because of the balance left over. i.e flight cost Eur48, paid with Eur50 voucher, so flight had 2 quid overpayment. But they do not refund this to you. I've booked similar flights before and they would usually show a zero balance. When I tried to check-in online, I got an error message saying "Money owed on flight, call reservation centre" They zeroised the balance and I was able to check-in.