I am almost sorry to revive this thread so long after it was initiated. I initiated a thread in the Folklore Society quarterly newsletter FLS News about eleven years ago which drew quite a lot of response before petering out.
So far as the original question is concerned. The truth about the Frittenden Treacle Mine in Kent is down to the geological fact that the Beult Valley in which it is situated comprises a very sticky clay soil. The 'mine' itself is in Dig Dog Lane (check that out, I didn't make that one up!) about a mile south of the village and almost within sight of Sissinghurst Castle. It comprises a worked out brickfield site. Over the last fifteen years, there have been unsuccessful attempts to have houses built there - all refused by the local council. There are also legends about the Frittenden Band Chaps, and the 'dock' to be build where the Medway was to link up with the Royal Military Canal.
Chris Rose (the Molly of the Seven Champions) told me the story behind the Tovil Treacle Mines near Maidstone lay in that the local paper industry was under threat during the Second World War because they were unable to import timber. As a solution to this, somebody tried to ferment straw, it later being found a sticky goo resulted. Chris supported this to explain the street name Straw Hill Road.
I am still trying to corroborate stories of treacle mines at Trottiscliffe (another place where gravel excavation is taking place), Lamberhurst, Tudeley and Challock.
Any takers?