I've just been given one of these DVD recorders with the same error, U81 (UART error) which I've now fixed.
The problem is caused by C1530 (100ÂµF 25v) on the Main PCB. This blows (open circuit), causing IC1508 (FET dc-dc converter) to fail. It's a tiny SOP-8 SMD on the underside of the board. This IC supplies D1R8V from the 12v line to the STi5516 processor board.
Replaced the cap with 100ÂµF 50v and cut and replaced the IC (MD5001T) and all works fine now.
You'll need to strip the whole box down to get to the parts, and be careful of static wrecking parts of the board. The story about wrecking the "Freeview Decoder Microprocessor" is frankly rubbish - unless it was wrecked by the engineer doing the repair.
Cost? About 1-2 pounds if you can get the parts.