Well, they went yesterday. I wasn't lucky enough to actually see them leave the nest but caught up with them immediately after, all sitting together on the nearest branch!
I've followed their progress since and they're happy to sit around and pose for photos! having looked in more detail, I think now they may be Great Tits. Originally, it was a pair of Blue Tits that built the nest but at some point, and without me seeing!, the Great Tits must have moved in!
I've noticed a large and noisy flock of Willow or Long Tailed Tits. The thing that makes me think they're Willow Tits is the 'v' in the end of the tail, but you can never tell with juvies! Will post photos of the later this week!