Does anyone have any experience with these? I had a go in a Honda Pilot years ago on a dirt track and thought it was great fun - I've now found that you can buy road legal ones, that still have chunky tyres and the stripped out style frame to them.

Blitzworld do a version which can be self built using doner parts from a Fiat - but it will take a variety of engines, so performance wise the sky is the limit. I'm not suggesting run out there and create a track day weapon as these are not the most aero-dynamic vehicles, nor the most safe, but with a decent engine you could get some cheap thrills. Combine with the chunky tyres and you might have an alternative to the typical track day car.

Can be built from around £5k, so not that expensive when compared to a caterham!

I haven't, but I seem to remember a programme which featured them being tested on roads (one of the "sensible" car progs, not TG!) and they concluded that they were fine off-road, but positively dangerous "on road" as they weren't very good at corners - seems there have been a few injuries with them....

When you say a sensible car program, could this be a clip I may find on youtube perhaps?

I've seen quite a few different styles, most of which have a "brush cage" which wouldn't protect you in the event of a crash. There are one or two out there with a more substantial frame surrounding the cabin which manufacturers do describe as a roll cage. Still, it won't prevent whatever it is that you have crashed into coming into the cabin - would only protect you from being crushed if it rolled.

Seen a few of them kicking around the roads, both fairly new looking and some ratty old things. Noticed the drivers always had helmets on so maybe there's some ruling there (treating them like a quad bike maybe?)..