Thank you CM. I will be thinking of a cover for winter. I have noticed that the heap is a little steamy now (condensed water on the insides) so I think they are OK for heat at the moment.
"Grandad" has two much smaller containers, and harvests enough worms for fishing and a bit of composting, so am not too worried about the size of these. But his tubs are opaque. So yes, that's the first thing I might change.
I have put the tubs into the dark corner of the garden so there is no sunlight. I am checking the moisture levels. At the moment, it's a bit like have a cross between guests and pets in the garden.