Update on this.
The blue tit I assume to be sat on eggs now. She's spending most of the time in the nestbox and coming out for very short periods only (like 60 seconds or so every couple of hours) Have also seen the male occasionally going in with food for her and flying straight back out.
The blackbird eggs must have hatched already as the female is making frequent trips with a beakful of worms and insects, as is the male although less frequently.
Sadder news on the Robin nest, which appears to have been abandoned. I am still seeing Robins in the garden but none have visited the site of the nest for about a week now. I assume they have built an alternative elsewhere (or maybe one has died

)
I found a Blue Tit nest this morning, in a crack at the base plate of one of those big road side concrete lights - saw it coming out as I ran past
I’ve found some nests in really weird places over the years :
The fag box outside a pub - Blue Tit.
Oyster Catcher on a patch of gravel outside rugby changing rooms.
Robin in an overturned paint tin on the floor.
I kestrels nest in an old crows nest, directly below was a wood pigeons nest and directly below that was a robins nest
A magpies nest, in the metal framework directly above the electric cables on a railway line - the noise must’ve been incredible.
Robins nest in a pair of old trainers in a shed.
A swallows nest on the security light in a front porch.
A full herring gull colony on the roof of an industrial estate by us - the lads in there hate the noise
Mallards nests in trees - work that one out !
Peregrines nesting on a ledge in the Jaguar Landrover factory.
Kestrels nesting inside a railway maintenance shed - the lads loved them and they had their own Facebook page
I’m sure there’s more, that’s just off the top of my head !