well, first off, if you want to get a new MOBO, try newegg.com. Their prices are pretty reasonable. I don't know if they ship to the UK or not, but it's worth finding out.
it could also be a PSU or even a simple power switch problem (it also sounds kind of like a short in the system) are there any beep codes when this happens, or does it just go straight off?
My Computer: eMachines T5226 - Intel Pentium D Dual Core @ 3 GHz - Diamond Viper Radeon HD3870 OC Edition with 1 GB GDDR3 VRAM - 4 GB PQi Turbo DDR2-533 RAM - Corsair 650W PSU with 52A on single rail - TSSTcorp DVD-DL+-RW combo drive - Lite-on 52x CD-RW burner - Creative SoundBlaster X-Fi XtremeGamer - Logitech z5300e speakers - WinFast TV Tuner - Vista Ultimate 64-Bit. eBay name: Flightsimboy304