Halfrauds 150 pce kit is a pretty reasonable starting point - when it's on special offer at 99 quid. Only a madman would pay the full asking price.

It will duplicate your spanners but contains pretty much everything else in terms of hand tools and it avoids a lot of shopping around just to buy everything in one box!
Failing that, I'd say you need:
1/4" drive socket set
3/8" drive socket set
Set of male and female torx sockets and bits
1/2" socket set and 18" breaker bar if you're going to tackle heavier jobs such as suspension work
In addition to either of the above:
Torque wrench that will cover down to 8nm (halfords Pro one is reasonable)
Heftier torque wrench if you're going to be doing suspension work, etc.
This is a pretty reasonable set of torx sockets if you've already got a socket set or two:
http://www.machinemart.co.uk/shop/product/details/pro151-29-pce-metric-socket-set/path/torx-spline-and-star-toolsVAT free days at Machine mart coming up if you know someone who has an invite.

Best to buy the basics, do a few jobs and add to the collection as you need to rather than rush out and buy a load of tools only to find you didn't need some of them, IMHO.

Kevin