Well as a guide I paid £5700 at a dealer (no trade in) 6 months ago for my 04 with 55k and it probably wasn't as tidy as yours, I had to get the wheels refurbed and the arch lips de-bubbled to bring it up to what I would call a 9/10 car - so you must be looking around 6k not taking the plate into account.
Try this site for a value
How much is my car worth? Get a free car valuation on DesperateSeller.co.uk it seems pretty accurate.
Just make sure to add the mileage and then click the 'adjust mileage' button
not the 'get valuation' button or it doesn't adjust the mileage
Edit - I forgot to add that book prices don't take into account supply and demand, when I was looking for mine there was a total of something like 7 manual roadsters for sale on autotrader in the whole of the UK, and because I didn't want black this brought the total down to 4, the dealer I was buying from had done his homework and hence was prepared to hold out for the price he wanted which was over top book.
My point exactly!!! 5.7k for an 04 is top money. An 04 in mint 5k tops!