£334.49 + £290.99 = £625.48
Subtracting £474.00 gives £151.48. So why the hell does the agent want another £300???
As I've said, keep hassling - but you have to accept that (whatever your legal rights may be) if the agent won't forward your booking without extra money you're not going to get the flights booked. If you don't get any joy out of the agent, consider cutting your losses and booking direct with Thomas Cook (which, in my opinion, is the only place you should have booked in the first place) so that you get your holiday - but then take legal action against the agent to get your money back.
Thomas Cook:
http://www.flythomascook.com/Search.aspx
Online small claims website:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Chris