my nice new custom 200x100mm coolant header tank arrived today from Wilabb on ebay. Most standard tanks have much smaller outlets but I needed 23mm out to match the intake on the M50 Block. I also had an 8mm intake rather than the 10mm standard though I can't quite remember the logic for that decision. As it's only to take the expansion it should be more than enough. The overflow on the filler neck is also 8mm.
All the fittings including the filler neck are machined rather than the cheaper swagged ones.
The plan is to mount it in the battery box area though I'll probably have to go off centre or the bonnet hinge will get in the way of the filler cap.