There is a bus afaik to Stanton Drew village from Bristol Bus Station. Or you can walk it from the edge of Bristol (according to the bloke who had walked there I met a while ago ;)
If you are insane enough to walk to Stanton Drew (its a few miles), go via Maes Knoll hillfort.

Stoney Littleton can be reached via public transport within a mile or two. Train to Bath Spa, then a bus out to Wellow, then walking. Or a 14 mile (give or take) round walk from the centre of Bath, which from personal experience hurts...