spicejetter, are they a limited company? If so, PM me their details and I'll check some of the public records to see there's anything untoward about their solvency.
If I was in your position, I'd send them a recorded delivery letter stating that if full payment isn't received within 7 days from the date of the letter you'll issue a court claim with no further notifications. For £450 it'll only cost you £45 via the Moneyclaim Online website and is very easy to do.