Under the Sale of Goods Acts a product HAS to be fit purpose. The purpose in this case is to be able to receive DTT in your house not in Comet's bl**dy store. (Proline is a Comet Own Brand.)
Next point. Under the Act, for the first six monthsi is for the retailer to prove that a product ISN'T faulty NOT you prove that it IS. That is to say, it is for Comet to prove that it does work in your house not for you to prove that is doesn't.
However having said that, if you have other digital devices that do work and the Proiline doesn't it means that it is 'Not fit for purpose' - in your case. The fact that it works in Comet's store (With, as you say, all its amplifiers.) is niether here nor there.
If it was me. I'd go back, speak to the manager and be firm!
Under the law they have to give you a full refund. Don't take a credit note or similar rubbish because you may want to take your business to a company that deals with customer complaints better. Do not take any cr*p from them if they don't comply just go staight to your Local Small Claim Court - you WILL win.
Terry
April 2008