Looks too good to be sold for £500, but I've sent a message through Gumtree asking if I could go to view it, it's not too far from me.
It does say "call me" in the ad, but no phone number given, and the photos don't seem to support "needs a lot of work to get back on the road" either, but I'd be interested to see if I get any response. If it's genuine, I'm going to be severely tempted.
(edited some time later)
After a little investigation, it seems a bit suspicious. Ad mentions a steel chassis but the Rossa is a fibreglass monocoque, according to what I've found (I had forgotten all I'd ever known about the car). It's supposed to be 1982, but production didn't start till 1987, although it might be registered as a rebodied Mini which might explain that. It's been posted for 7 days already, apparently, and still unsold; if it's genuine, I'm surprised it's not been sold in that time...
...but the address could well be genuine, I've been looking around on Google Street Images and found what could be the drive it was photographed on.