I just had a new head unit fitted by MAS Audio in Northampton, I did most of it myself but didn't want to do a crap job on the ignition live wiring so I got them to wire it to the fuse box properly. If you don't want to do any yourself it might be worth a look. I think the general rate is 50 quid an hour for audio installs.
You won't get good sound from the stock head unit unless you add an amp though