Legally, the seller has the responsibility of replacing/repairing the motherboard if it is faulty/damaged when you received it. That said, I know how difficult it can be to get retailers to fulfill their obligations, short of threatening court action.
The following is taken from "Consumer Rights: Give me my money back at:
http://www.moneysavingexpert.com/shopping/consumer-rights-refunds-exchange#goodsKnow your rights
When returning items, beware shops trying the oldest trick in the book: saying they're not responsible for the shoddy goods and you must call the manufacturer. This is total nonsense!
If a company fobs you off by saying “go to the maker instead”, it's wrong. It's the retailer's job to sort it.
It doesn't matter if it's an iPod from a high street shop or a designer frock from a department store. If something's broken, torn, ripped or faulty, the seller has a legal duty to put it right as your contract is with them.
What proof do you need?
Since 2003, consumers have had extra benefit from The Sale And Supply of Goods To Consumers Regulations.
When goods are faulty, if you return them within six months then it's up to the shop to prove they weren't faulty when you bought them. After this, the burden of proof shifts and it's up to you to prove they were faulty when you bought them.
But that's not all.
There's another piece of legislation called the Limitations Act (it's the Prescription and Limitation Act in Scotland) that can help you out.
You have up to six years after you bought a good to complain
(In Scotland, it's five years after you first realised there was a problem)
This comes in handy if you buy goods, but don't use them for a few months or if something breaks after the six month rule and the fault was likely to have been there all along. Yet the longer it's been and the less durable the item, the harder it'll be to fight for a full refund. In such a case, you might have to accept a partial refund or credit note.
If they refuse point blank to help you then try contacting Gigabyte Technical Support here:
http://ggts.gigabyte.com/tech.asp?ClassID=2&Country=U.K.&SourceWeb=B2C It can take several days to get a reply so please be patient.