Well, I'm no expert, and bearing in mind I'm guessing from a buyers perspective, not a sellers, so I'm looking at it from a less involved viewpoint, I'd say at least £7000, but if it were on ebay, I think you could start it at £7000 and it would more than likely rise.
Its an awesome motor mate, what a sound!!
Are you taking it anywhere? Like any shows or anything? I'd love to see/hear it in the flesh.