Normally I would say don't bother buying worms as they will find their
own way in. In a plastic dustbin however, it may well be worth your
while to introduce them. Like others have already said, your heap is
decomposing anaerobically, which is why it stinks. Worms won't like
these conditions, and I would suggest that as well as adding cardboard,
newspaper, etc to the bin to increase aeration, making some holes in the
bottom to allow excess water to drain. I know you said it's not too wet,
but the bottom contents of the bin get saturated and rot anaerobically
if there is no drainage.

A bous to doing this is that compost worms will find their own way into
your bin when the conditions are right for them. I took the lid of one
of my compost bins last night, and the top was literally writhing with
worms. Lovely! And I haven't bought any.

HTH.

Right, when I get home tonight I shall take a hole poker to the plastic and
melt some holes in the bottom, see if we can get some air circulating in
there! Will also see if I can get some pipe and put some holes in there, add
some paper, cardboard and maybe turn the pile again.
Where would I buy worms from? I ask a guy at work who is a fisherman, if
they'd sell them in a fishing shop, but he said they might be the wrong type
of worms, is this true? Do garden centers sell worms? I haven't seen any!

Cheers

Ben