I never have trouble getting a good supply of compost worms into my bins so
it has raised a question. Why are compost worms for sale?

I believe that all of the Dalek type bins are open bottomed so even if they
are standing on concrete, worms will find the bin, look at when you lift a
wet paving slab or pot, there are worms underneath.

OR; and this is the second question. Are the compost type worms which I find
plentiful naturally, an element or characteristic of of the area 'South of
the Watford Gap' for example?

Mike