Wen
It doesn't matter if you allow Powerhouse to repair the TV, you do not lose any rights to have a full refund if the repair is unsuccessful. My original 28Z07B was supposedly repaired by the supplier's service agent that I purchased it off, but was even worse when they returned it, so I gave the company I bought it off a refund or replacement ultimatum and a new one arrived the next day, which thankfully is OK!
The shop you purchase from is also liable for the product for up to 6 years after the purchase date and remember that any guarantee is in addition to your legal rights and is offered by the manufacturer really as a gesture of goodwill as anything else. Do not let them fob you off with 'it is not Powerhouse's policy to exchange just repair' as they really do not have a leg to stand on.
If you purchased with a credit card (not debit or charge) you have even more support. If the item was £100 or more the credit card company has obligations to you as well, under the Consumer Credit Act 1974 with a principle known as 'Equal Liability'. This means that both the credit card company and the supplier have the same responsibilities to you for the goods being satisfactory. If you purchased with a credit card complain to the CC company as well.
Failing all this, the last resort is the small claims court, where more info can be found on
www.courtservice.gov.uk
All your rights and more can be found on
www.tradingstandards.gov.uk
Hope this helps.
AJ