AIBU to ask my next door neighbours not to smoke indoors?

It depends on how your house is built. Mine is a modern terrace and you can definitely smell it very strongly. So much so I could tell when he was lighting up a new one, he chain smokes. Sometimes stays up all night and smokes.
There is no way I am putting up with that. If I can smell it it means I am breathing in toxic particles and I don't want lung cancer from someone else's cigarette smoke.
I don't care about neighbourly relations, if someone is making my life a misery with their stinking cigarettes then they are not good neighbours and they don't care either.

Could the council really do anything to stop someone doing something perfectly legal in a house they own?

You can ask but I'm afraid I don't think it's reasonable to try and control what others do in their own home. If they were to do so outside wouldn't it blow into your garden/windows potentially anyway?

I'd not be happy if someone expected this of me. I'd be having sympathy if the houses are designed in such a way that it disturbs others but behind closed doors really is ones own private domain.

No, that’s not at all a reasonable thing to ask of them.

I understand how porous these old houses are, though. I can smell when my next-door neighbours have burnt their toast.

Well I certainly did. My new neighbours moved in and stunk my house out 24 hours a day with their disgusting cigarettes.
I once went round there at three in the morning and said can you stop smoking next to my bedroom because it stinks and I'm working tomorrow. I hadn't had any proper sleep for weeks because of their stench.
So they stopped smoking in the house.
I'm not putting up with that.
If they refuse you can do something - you can get a special air purifier which will take it out of the air and reduce the smell, it must have a carbon filter. The one I looked at was about £500 because it needed to cover quite a large area.
Or you can complain to the council who will tell you what you need to do to prove your complaint, and environmental health if it's affecting your health.

I live in between two smoking mostly indoors homes and it baffles me people on MN can smell it through solid wall, I cant smell it at all! Even when we all have our windows open.

I dont think its unreasonable to want to ask if you can genuinely smell it. But you do need to be prepared to get told to piss off (or similar) and for it to ruin neighbourly relations in future.

Air vents, chimneys, broken skirting and so on. Find the source and plug the gap.

I wouldn't ask them to stop, since I reckon they will tell you to F off. As pp said, cooking smells, BBQs, and so on are part of sharing walls in homes.

Would a good electric fan to outside help. Like in a bathroom with no windows.

Yes you can ask, they can say no.

Improve your wall or whatever you need to improve.

Leave your neighbours alone.

I don't think you can ask them not to smoke in their own house - it probably says something about the way the acceptability of smoking in our society has shifted that someone would even consider this as an option.

I would find a way of sealing off the living room from your side. It should be preventable.

If they are good neighbours they will also work with you to try and help find the leaks.

I totally get that it's annoying but it would be unreasonable.

I smell cooking every day from my next door neighbour. Sometimes it's nice, sometimes very much not so. I can also smell - sometimes - when they go to the toilet, as our bathrooms are connected.

I can't tell them to stop eating though.