Daz,
It really does depend on what pattern you want built, a cross or radially spoked.
If a cross then I think in the order of 210mm not sure what that is in old money though, that said I am building with a high flange hub in a cross pattern, certainly for the rear anyway.
I have a box of brand spanking 238mm silver halo spokes - if these will work on your hub and rim combo let me know. It might be worth putting a call into Alans to see if they can advise?
Let us know what you find out.
Paul