Well, it seems that there are 4x different upper arm part numbers for each side. I haven't got to the bottom of exactly which is which nor exact chassis / year splits but now know enough to order the correct ones for my car.
For the LH side the details I do have are:-
Up to chassis 7019303 - part no 50704947
From chassis 7019303 up to ?? - part no 51787337 (This includes my 07 plate.)
From chassis ?? to chassis ?? - part no 51835397
From chassis ?? (15/02/2009) part no 51817623
I'm not sure if that is helpful but it is a starting point.
There are basically two types of upper wishbone: early type are for cars with steel uprights, and late type (with the slimmer balljoint taper) are for cars with aluminium uprights. This changeover happened sometime in 2008, and if you're not sure what material your uprights are, check them with a magnet; it won't stick to the aluminium ones.
And a note of caution: the chassis number cannot be relied on 100% as I've had several cars in the workshop which eper showed as having steel uprights when they were in fact aluminium.