OP "partner has love/hate relationship with mum, he felt caught in middle of it all and just wanted to keep the peace."
How can he be caught in the middle! He can see her unreasonable behaviour towards his wife, there is no middle.
"...think mil has a lot of 'issues' and he's learned over time that to ignore madness is the best policy. although this didn't really help me at the time..."
It certainly doesn't help you. So if this continues just tell MIL no, no I
don;t want calls in the middle of the night, or doors or windows banging, I won't answer unless you telephone ahead at a reasonable time.
Put to rest these memories of baby's first year and just move on. You
have shared these with us and we seem to all agree it is really unacceptable.
So just decide if it happens again, no way, you won't allow it.
Regarding your dh "... it caused a lot of stress between us at the time...which I think is precisely what she wanted (ie some attention). very very very frustrating for me as it was classic it's alright for him to slag her off but noone else and at end of day he's her mum. I still get frustrated now thinking about it!"
Decide how you want to proceed. Pointing out reality is not slagging someone off. Concentrate on getting the results you want. Not debating the issue, just what will you two do/say together to her, and about this. No slagging off needed. He let you down, and if you can forgive him move on.
"... be civil but detached from her..."
"...but it's really hard at times."
I think you need to decide how hard it is, if it is too hard just tell your husband how much his mum upset you and that you will be avoiding her. His job is to protect you from her, he is not in the fucking middle, he should be in front of you, defending you.
"...with regards his mum he's seen it all and worse before so it's not his fault." I am not sure this follows, if he knows how she can be it is his job to protect you, unless he is genuinely scared of his own mum. It is very possible having experienced her for years he is somewhat paralyzed when it comes to her and feels unable to stand up to her.
If this is the case he needs some assertiveness training or counselling to help him to deal with her.
The civil but polite is fine if you can do it. If you can't, you need to work out what you can manage.