Thanks for that. I think that I was 'reading your post backwards' (if that makes sense!). I've only just worked out what the true situation is.
Any householder has the right to determine who does, or who does not, enter their home, but the law doesn't really offer any solutions when one householder (i.e. your father) wants to bar someone from entering but the other householder wants to let them in. (To be fair to our law makers, they could never produce laws which covered every possible domestic situation).
If you call at the house when your father is there he has the legal right to prevent you from entering his home but, as long as your mother agrees to you entering, I can see no reason why you can't visit when he's not there. (Obviously that doesn't help much if you've not got a key and your mother can't get to the door, but I'm simply pointing out that any rule he makes can be over-ridden by your mother when he's not there, since they both have equal rights to determine who can or can't enter their home).
I suggest that your best approach might have nothing to do with legal considerations. You need to seek an intermediary who can get your father to appreciate that it's in his wife's best interests to allow you to visit your mother. If there's no family member that can assist, it might be worth trying to get support for your mother through a relevant charity (who could get someone to visit her) and then try to get the support worker to make your father see sense.
Chris