An Advent 5421 is not a Motherboard (even though it contains one), it's a notebook computer model!
What problem are you having that you think a bios update will fix?
Advent is the in-store brand of PC World (and there is zero support from PC World). The 5421 is basically a rebadged Uniwill U50SI1 (you should see this printed on the underside of the laptop). I'm fairly certain this is the same as the Gericom Phantom A4 Superlight 53120
, the Gericom website contains drivers and a user manual however, no bios files are posted there. I suspect that finding a bios update for this notebook model will have to come from a custom bios house.
Advent Repairs and Spares:
All official service and support is handled by The Tech Guys (http://www.thetechguys.com/)
, if your laptop is covered under warranty then you need to contact them for repairs.
If your laptop is out of warranty then there are a number of other companies who do either repairs or supply spare parts, here are a selection -
TechnoMart - http://www.techno-mart.co.uk
DriveMyLaptop (replacement drives) - http://www.drivemylaptop.co.uk
Laptop Solutions UK - http://www.laptopsolutionsuk.co.uk