Advent are predominantly made by a company called 'Centreprise', based in Basingstoke.
They are build specifically for DSG.
Sometimes alternate PC assembly companies are used to help keep up with demand.
The parts are normally quite generic, if not a little cheap and nasty, so getting it replaced shouldn't be too difficult.
Are you sure it's a mainboard problem?
If so, replace it with a board that will take your existing componants.
Don't forget, if you're on Windows XP, changing the board (Northbridge) requires a complete software re-install.
Redders, March 2008