My favorite Steyr supplier is Hans Manders from http://www.goedschot.nl/steyr/
He is from the Netherlands and I do not know if his English is any good but I know he does sell spare parts to the UK. And his prices are really good. He's a very good technician so he can help you out with that as well if you have any questions. He is an official Steyr dealer and can supply any rifle and spare part that Steyr makes.
Send him an email: firstname.lastname@example.org