There are lots of clubs that hold events and if your prepared to travel then there are events most weekends somewhere - you can start with a pretty much 'stock' car (we did) but then when you get into it the modifying is just as much fun as the competing.
Often stuff on E bay all ready prepped - there is a Marlin on there at the moment, just search Trials Car.....
Me and Dan were signed up for the Exeter Trial about 5 years ago but it was the year they cancelled it the day before because if that snow and ice thankfully!
Thanks for all the info.
I'll be in touch once work has died down! Not too sure about a Marlin though, it'll need a large shoe horn to get me in and out!!
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Culmstock area - Devon TRF Group member - KTM 690 Enduro