Whilst I was looking to replace the standard headunit in me astra, I searched high and low all over the web, so I thought if anyone else was looking, you could have a browse around the following sites:
Bass Junkies: http://www.bassjunkies.com/
Car Audio Centre: http://www.caraudiocentre.co.uk/
Car Audio Direct: http://www.caraudiodirect.com/
Car Audio Security: http://www.caraudiosecurity.com/
HiWay Hifi: http://www.hiwayhifi.com/
Lets Buy Car Audio: http://www.letsbuycaraudio.com
New Image Car Hifi: http://www.newimagecarhifi.co.uk/
There might be others, but I havent come across any.
The only site that I can comment on is "Lets Buy Car Audio" I bought my headunit from there, payment simple, delevery was fast, next day if I can remember and staff friendly Oh and was the cheapest I could find at the time.
They seem to have up'd the prices a little, proberbly becuase of xmas.
Hope it helps someone out there!