Geoff, if you have Facebook there is the Superkart Drivers group where many of us are active between race meetings, we don't bite.. honest
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https://www.facebook.com/groups/534983186517728/
Also there is British Superkart site that has all the latest regs / results / for sale etc -
http://www.superkart.org.uk/
Superkarting UK is a club at Darley Moor which do race schools and let you have a go in various karts, 125cc being 1 of them and also you can do your Arks test there too
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http://www.superkarting-uk.com/Race%20school.html
To help you out a little there are 2 125cc Gearbox classes. There is 125 Open which is mainly on long circuits which you can run any 125cc Kart Engine from current engines all the way back to early 90's, often on a long circuit style chassis (more laid down seating) with fibreglass bodywork.
The other 125 class is KZ2 which is mainly short circuit which is the normal short circuit style kart with plastic bodywork but restricted to more modern engines that have been through the last 2 / 3 homologation periods.
My advice is don't jump after something this minute and pick up the wrong kit, and stay off Ebay!
If wouldn't just restrict yourself to 125's, there are also 250's and 450 4/'s which are popular too. 450 also has the appeal of being electric start too!