Thanks for all the wise words. I'm just letting Alfie come and go as he pleases at the moment. He seems quite happy as he's being quite vocal and he gets lots of exercise when he's out of the cage because he keeps flying round the room. He's fascinated by the 'new' cat tree (we got it in December) so keeps flying towards it but circling back... he'll do this several times in the evenings. He used to spend a fair bit of time on the old one and I think he's keen to do the same with this new one but has only actually landed on it once. He has his two shelves which are full of toys and paper/card to shred. I got him a new foraging toy and hung that near his shelves so he often grabs himself a walnut or banana chip from that.
He's still good at getting in his cage to go to sleep - I have to use a perch to fetch him from his shelf (I'm a little too short to reach the higher shelf!) but he often steps up with no problem and climbs into his cage without issue. I give him lots of praise but have held off offering a treat in case he lunges at me. I want to ensure I am able to offer treats with confidence. I think if I approach whilst feeling nervous he will pick up on that and it's more likely to result in a lunge.
I also (finally) got him a UVA/UVB light this week which he seems to enjoy sitting under. I don't think that will necessarily make a difference in terms of behaviour, but hopefully it will help beat the winter blues. (It's always very dull, grey and miserable here during winter)