They are the same fitment as a Jeep Grand Cherokee - 1/2 inch thread, 20mm long. Halfords do a locking wheel nut - do a search on this for the part number - I got mine around a year ago so search back to then.
I never thought to look to see if my new Tamora has locking wheels nuts. I wonder why TVR doesn't supply this from new when even my ickle C2 VTR comes with them as standard?
Locking wheel nuts was the only nice suprise I had in dabbling with Ebay! Bought a set for £14 (I think!), and they fit just fine!, the Shiny Nuts however, were £80! lol, look great though