It's called a motherboard for exactly the same reason they are called motherships. The main part that little bits come out of.
If it is a dead motherboard, then I'd be rather surprised if it got repaired for 40 quid.
Actually, I'm not sure their advertising is really all that honest.
Fixed labour price is fine, and realistic, since for more than 95% of problems a competent fixer can sort it in less than an hour.
Their economy with the truth is in new parts.
A MB varies in price a lot, for a home PC I guess that's 20-60 quid.
But it might be the hard disk, power supply, or if you're unlucky they may say it's more than one part. Happens but is very unlikely.
If it requires a part, they will want to charge you for it, and may want extra dosh for labour.
You will have to pay their price for the part. They are very coy about this, indeed I looked about and couldn't find that at all.
You may think they play the averages, but given that the MB on my PC cost >200 quid, and they offer to fix it, for much less, it's not plausible.
MBs in modern PCs die quite rarely, my guess is something coming loose, since that's the most common sort of problem, and pretty easy to fix, so you get decent odds that it will only cost the stated price.
I find a few other things make me trust them less. They don't talk about charges for shipping the PC back (or it's not obvious at first glance).
They use a premium rate line.