Lots more checking.. it now seems that the difference between the SRX4 / 6 and the FZR250 fork pitch is 10mm. Not that it makes a lot of difference in the grand scheme of things. The Front axle is the same diameter (same wheel bearings), no doubt the hub on the SRX wheel is wider but that is easy to resolve. I will need a new front axle as at 5mm it may have been possible to use the original FZR item but 10mm is too much and I have sourced the correct item and it is exactly 10mm longer than the FZR250. Going this way may hold up the front end a little.. but I have other work I can get on with while I wait for some stuff to come from Japan.