Maybe more to eat at the top and they have finished munching at the bottom of the pile

More likely than not they are mating to make more worms for your compost.

Is this a compost heap or a wormery?

Worms are actually very social creatures. They surface to meet & greet each other and of course, to make babies. Usually at night, or when it rains they will surface so they don't drown.

Thanks everyone.
Gnarly Gnome, I wish my compost came near to the top of the bin, but no it's still less than halfway filled.
Bamboo, it's a council bin but I did have a wormery at one time.
Loosestrife and Bathgate, that's a nice thought that they come up to mate. Actually, that's news to me 'cos I thought worms are hermaphrodrites. Yes I can tell they like being together by the way they snuggle up in great heaps.

I leave the lid on most of the time. Should I take it off?

Hi Merlin

simultaneous hermaphrodites so the worms need each other to produce eggs....

If me I leave the lid on unless its too dry ! I have a council dalek comes in handy

Thanks for the info and advice Gnarly Gnome. I'll take the lid off more in future.

I'd be concerned if you have a great number of worms on the top layer or, worse, on the sides of the bin or congregating in the lid. It can indicates an imbalance in the heap making it an uncomfortable environment for worms. That might be because its too acidic, or way too dry - does it smell awful when you open the lid, or is it fairly sweet smelling? Is it very, very wet or very, very dry? Do you turn it at all?