Thanks both. Trawling ebay came up with these:
https://www.ebay.co.uk/itm/351109003241 and
https://www.ebay.co.uk/itm/311002137329Checking specs, the VF400 has a 35mm fork stanchion the same as the XBR. What does the team think?
Just rang Ecosse Motorcycles who're waiting on fork seals but tell me the bike should be ready this week. TBH rather they did seals MoT etc then I can hit the ground (running or not - if the Truss reference is allowed!)
I've been mulling over tank options which seem to me the main problem with my project. I have had a thought - I don't want to make irreversible changes to the frame. However, one of the problems will be the location of the bobbins on the sides. If I can find a tank which is close, I could perhaps make new eccentric bobbins to mate up with the cups and secure them with another screw into a tapped hole. But I need to see what's what first.
As far as the seat is concerned what I may do is get another XBR seat and rebuild the base with a forward extension to meet what will be a shorter tank, and maybe chop off a bit at the back. Then possibly give it to Leightons to make foam and cover - or do it myself.
For both these I need to bear in mind Sherlock Holmes' maxim - it is a capital error to theorise in advance of the facts.