What's he up here for, anyway?

It's not polite. Sir George is a respected businessman entitled to his views. I may think that Sir George is wrong but that does not allow me to resort to abuse. (I only do that with politicians.) The BBC reports:

Prime Minister Tony Blair has dismissed a top businessman's support for the Scottish National Party as "self-indulgent and absurd".
Sir George Mathewson, former Royal Bank of Scotland chairman, announced his support in a letter to The Scotsman.
He backed SNP leader Alex Salmond for first minister and argued that the SNP and independence could liberate Scotland from a "dependency culture".

Furthermore, the Prime Minister is in no position to call anyone else 'self-indulgent'.