Matt,
I'd written you a beautifully long and detailed PM, then hit the back button on my browser and lost the lot! In a nutshell:
There's no 'kit' of B4 and Pro-Kit springs. B6 are very hard but designed for full length springs, the B8 are the equivalent for lowering springs but IMO they're way too hard for normal road use. The B12 kit is, AFAIK, B8 dampers with Pro-Kit springs.
I found the part numbers on the Bilstein and Eibach websites, then googled to find the cheapest retailers - all of which happened to be on eBay.
The springs came from Demon Tweeks' eBay shop:
http://myworld.ebay.co.uk/demontweek...id=p4340.l2559
The dampers all came from this guy:
http://myworld.ebay.co.uk/suspension...84.m1439.l2754. Great customer service, parts came direct from Bilstein but very very fast, so he must be switched on with regards to ordering.
Still very pleased with the ride, can't fault it at all
HTH