What type of mining are the following English towns known for: Bisham, Tadley, Pudsey and Dunchideock?
I should add that, so far as I am aware, none of these mines has actually produced the thing for which they are famous.

That'd be treacle mining !

Well done Dogspit,
Yes, it's the fictitious practice of treacle, or molasses, mining.

Wikipedia offers a couple of explanarions for the joke:

"Treacle" originally meant any kind of a thick syrupy salve, and it is likely that bituminous seeps from coal deposits were used in traditional remedies, so this may have been the kernel of truth that inspired the joke. The Tar Tunnel near Blists Hill in Shropshire has natural deposits of tar oozing from the walls which could be said to resemble treacle.
Another explanation is that "treacle" originally meant 'a medicine', derived from the appearance of the Greek derivative 'theriacal' meaning medicinal (Gk theriake = a curative or antidote), so the various healing wells around Britain were called "treacle wells". Treacle later came to mean a sticky syrup after the popularity of a honey-based drug called "Venice treacle", and the continued use of the old form in the treacle wells led to the joke.

Aside from the towns listed above, there are numerous other treacle mines in the UK:

Treacleminer

And as a piece of language trivia:treacle comes from 'wild animal' in Greek . The Greeks took their word theriake [ see dancegirl's post ], a salve, curative, antidote from their word therion, meaning a wild animal.Theriake means 'relating to a wild animal' It's the feminine form of the adjective from therion. The adjective became used as a noun on its own.It was originally descriptive of a particular kind of sticky substance. That's all because that treacly, sticky, substance was smeared by the Greeks to the wounds that they would get if gored by a wild boar or bitten by some other wild animal.