> Are there any sites in the South East Wales/ borders
> area that are accessible via public transport?

Welcome aboard sttomas!

I'm based in mid-Wales, but I have a car, so I tend to use that for visiting sites. I reckon there must be a few sites that you could potentially get to by bus though (possibly with a bit of walking as well).

Tinkinswood and St. Lythans must be near a bus route?

http://www.themodernantiquarian.com/site/110
http://www.themodernantiquarian.com/site/109

K x