I spoke to the owner about this...
...Before buying my car, he knew little about the car, except that it had a middlehurst 500bhp which 'apparently everyone knows' produces about 550bhp but when a button was pressed it gave 60bhp extra apparently because it ran more boost... (ie a splash boost botton)! The owner knew little of the work as it was done by the previous owner, could tell me about whether it was a stock bottom end, what pistons were used etc...
No dyno proof and as far as I could tell it was running 2510's so I think the figure is a little out
Still £18k for a 500bhp is pretty good, only thing to watch is I think the conversion was done at about 15k so maybe due some TLC...