Sockets - add 22mm (V6 coolant bridge, etc). Seem to recall needing a 21mm on a suspension component once, can't recall what though.
Also - you can get by without them, but slightly offset torx spanners can be a godsend, especially on the front coolant transfer pipe retaining bolt.
In addition, a good range of pliers, incl long nose/needle nose, hose clip pliers (for the cooling system and SAI pipes), water pump pliers are useful too - and of course a set of fuel filter pliers for petrol models - especailly Estates.
Timing setup kit for concentric idler setup on V6 models (£60 off ebay, fine for DIY use)
Rubber mallet - for those awkward pulley's and water pump!
Device for sucking oil out of plug wells - often overlooked!
Wire coat hanger or simelar, for cleaning breather assembly
24mm open end spanner - for EGR wiggly pipe and holding camshafts still
Angle measuring gauge - head bolts and simelar
Cheapo Tech2 for code clearing - especially useful on DBW engines.
For the more adventurous - valve spring compressor for head rebuilds (Halfords laser one at £30 is excellent)
As odd as it sounds - a gunson timing light has come in useful for me a few times - not to check timing, but purely to identify if HT leads are carrying a spark, on troublesome cars
Cooling header tank cap with Shrader valve bodged in - for pressuring coolant system with plenum off, and identifying leaks
Sump plug helicoil kit - a must have on older models with the hex drain plug!
A hand-pump from machine mart (£7 ish) for topping up autoboxes. Alternatively, for filling autoboxes, a "killaspray" type device, around £30 from a garden centre, is a great ATF filling method.
Some 3 tonne sturdy axle stands and a good, strong trolley jack, are a must. Folding stands are to be avoided at all costs. I recommend the machine mart, "Clarke" heavy duty 3 tonne jack, at £80. Stands can be had from halfrauds for £30 - combination works well for me.
A mirror, slightly offset on a little pole, for looking down back of engine
Easibleed - for reverse bleeding manual transmissions..