They're really meh guns to shoot. I know most would never even gas them up but I had a 6" and 10" versions and they were a disappointment. That's the old TM DE for you though I suppose. Cool looking grips though.
Even with their rarity, I think £500 is a stretch.
Step one: buy a SixG brass super nub (best upgrade in my eyes)
step two: buy a modify tan bucking
step three: buy some geoffs .36 bb’s
step four: fit the parts and test
step five: return here to tell me I was right lol