The description in Mendip Underground is very good (apart from one crucial step). Park on the grassy verge just down from the junction to the farm. You may then have to walk back up the road, away from the farm, to find a grassy track leading into the trees on your right (this is the step that is missing from MU). Follow this track to the end, hop the locked gate at the end and turn right into a field. Follow the field boundary round to a stile, cross the stile, and head down into the woods. Cross a wooden bridge. Very shortly after the bridge (20m) is the big concrete entrance to Longwood Swallet.
In low water conditions 3x15m rope and 1 15m ladder (plus a few slings and krabs) would be sufficient to get you to the main stream. One rope on the climb after all the tight stuff (MU says a ladder can be used here), one rope to protect traverse into the Great Chamber and finally one rope to life line the ladder down Swing pitch.