Premium Code Not Validating

[AlanBarrett]

I received a 6 month premium voucher in November which was to be vaid up untill 31st May 2016 but when i tried to use it today I received a message saying "Unfortunately this Premium code does not seem to be valid. Try again or contact customer service." The code is inputted exactly as was written, I've read and re-read it. Anyone know what the problem is?

Thanks.