Thanks for all the input, guys.
I'll have a look at the spoke lacing tomorrow. So you're saying it could account for the perceived offset if it's wrong ? The outfit that did the wheels are more at home with British classics than Jap ones, so who knows !
Right, so it's not an 'F' - that's reassuring.
Finally, I like the suggestion that it'll tweak or spring by 5mm or so. I can actually move it by hand a bit but of course it springs back.
I've got a hefty bottle jack and will set that up to give it some 'persuasion'.
I like the chain idea to check the alignment, also the wheel check but as yet, the motor's not in the frame, so the chain's a non-starter but as soon as I get the back wheel in I'll set up a long straightedge.
Many thanks once again. Usual disclaimers accepted.
Oh, talking of disclaimers, can I mention great service I've had recently from Westfield Fasteners for stainless bolts and from UK Bearings for small quantities of shims in various thicknesses. No connections and usual disclaimers !