After posting last night that I hadn't received my money back, guess what - a cheque in the post today with a letter apologising! And to think I doubted them! The letter is from a company called Independent Telecom Customer Care Service - they apologise for the problems I have experienced with one of their clients text services, so I still don't know the exact name of the company who sent me the text.
If it's any use to anyone else, their phone number is 0871 231 6604 and their email is help@need-help.co.uk