thanks Taz for the bird info. They are not solitary (pity) so they must indeed be rooks. Everyone round here calls them crows, unless they are at the Tower of London in which case they are ravens.
These are big, raucous and numerous. I must pay more attention to the beak colour next time.
They do seem bright, but I find them quite sinister when they sit en masse on the heath.
And yes - the couple must be mistle thrushes! I'll email my neighbour (this is how we communicate in London: sigh) as he asked what they were as well. I'm sure they have a nest in our garden's smallest tree.
My parakeet friends are back too

Wish I knew more about birds as I sit up here in the top flat and can hear loads of birdsong all day.
PS a neighbour has a quite realistic looking, fake owl sitting high in a tree in front of her flat to deter pigeons. Don't know if that would work for rooks?!! maybe not.