People like BTM performance/fred (Birmingham) TDF (london ish) Birchdown (no idea, not southcoast though) Rentech (portsmouth) etc always get recommended but there are other specialists. Have a look in the traders section.
I found K-tec in the New Forest to be the cheapest at £420ish inc everything but £500 seems to be an average price.
Given the amount of problems this forum sees due to dodgy cambelt changes, it would appear a specialist is a very good bet.
EDIT: It's usually more like £350 to £450 if you have a model without climate control such as the 172 cup.