There's no law that says you can't put latches on so the doors can't be opened from the outside, is there? As long as you let him in? I know the law is stupid about having to grant access if he's on the lease, but if the doors are latched at least he can't surprise you by walking in. Also, are you within your rights to deny him entry until it is convenient for you (ie you can have a third party there) or does the law specify that access must be granted whenever he says?
The standing outside would certainly be unnerving! But, and again laws may be different in the UK, at what point does this behaviour become stalking or harassing? It's one thing to say "I am legally entitled to enter the property" and quite another to say "I have the right to stand outside simply to make you uncomfortable". I'm sure there's a threshold for stalking, I'd talk to the police to see what that threshold is. In the meantime, keep a log of when it happens & how long it lasts (I'm sure you already are). Do you think he's doing it hoping that DD will see him and cry for him so you'll let him in?
Just a thought..if your LL were to change the terms of your lease in some way, would that mean a new contract? If so, maybe he'd be able to drop him off the lease. IDK, like changing to 'no pets' (if you don't have any) or 'no subletting', even a £5 rent increase? (Grasping at straws here)
It will fade in time, that stupid feeling that he's standing at your shoulder. But any steps you can take (even tiny ones) to push him away from your 'space' will help the process!
Mine was the fact he made me feel I didn't deserve any good things. I've told this before, but I went to the store after the split and saw some cute 'fancy' hair pins. I wanted them so badly, but felt I didn't 'deserve' them. I stood there for what seemed like ages, arguing with myself. I finally grabbed them, guiltily threw them in my basket and headed to the till. I fully expected 'someone' would shout "You can't have those!!!" but of course no one did. It was really a 'breakthrough' moment for me, I realized that I DID deserve good things and that I was entitled to happiness. You'll have your moment too, where he suddenly ceases to 'haunt' you.