Dont know what the seat is, but I dont think it is a Norton part. The tank could be a Norvil racing part. The brake caliper looks like a Lockheed caliper mounted on an adaptor plate with the original front disc.
Here is a picture of a Fastback, borrowed off Les Emery's website
http://www.norvilmotorcycle.co.uk/
This is a newly built bike, the original Fastback would have had a two leading shoe drum brake on the front.
The only reason I dont have one like this is 'cos (yet again) I failed to win the lottery last week.