To have told NDN that dh won’t be helping her anymore?

No, I wouldn't check his phone either OP, but I would be asking him what the matter is - and not letting him get away with fobbing you off.
If he honestly can't see how her crying to him behind your back (trying to come between you), calling you unkind, him defending her to you, instead of defending you to her, and sulking shows that there is something going on, then he's being a disingenuous dick. They may not be having an affair, but there does seem to be some sort of emotional attachment between them which could lead to a physical one. Your DH needs to be reminded of where his responsibilities lie - with you and the children he helped create. In short, he needs to grow the fuck up, remember he's a husband and father, and his family should be his priority.

She sounds young, immature & a bit overwhelmed tbh. And your husband sounds like he's enjoying white knighting & being needed by her. Even if she were married, if her husband was working, she'd have to manage these things daily herself, while they were at work. I'd be upset at the messaging him about you being unkind & that she's been crying, that's bonkers, and a closer emotional relationship than she should have with a neighbours husband. Who on earth messages someone about their wife, like that. He's a fool if he thinks sulking about it is an acceptable response & should really examine why he feels like her messaging him in that way was appropriate. Even if he wanted to help her & disagreed with you, he should've been stamping that behaviour out immediately. But he hasn't & that's the issue.

@CurlewKate , are you really that naïve that you can't see the red flags?

It really isn't a case of "The point is, if I didn’t trust my dp to be able to carry a woman’s pram upstairs without shagging her, my relationship would be over."

No, I wouldn't check his phone either OP, but I would be asking him what the matter is - and not letting him get away with fobbing you off.
If he honestly can't see how her crying to him behind your back (trying to come between you), calling you unkind, him defending her to you, instead of defending you to her, and sulking shows that there is something going on, then he's being a disingenuous dick. They may not be having an affair, but there does seem to be some sort of emotional attachment between them which could lead to a physical one. Your DH needs to be reminded of where his responsibilities lie - with you and the children he helped create. In short, he needs to grow the fuck up, remember he's a husband and father, and his family should be his priority.

Or it could just be a sign you’re sick to death of your neighbour keep pestering you and don’t want to keep answering the door to her.

Of course, you would need to take some time to work out whether your concerns were well founded or not before broaching the subject. That isn't really what I'm talking about though.

If I felt that I had to ban my DH from helping a neighbour because he couldn't be trusted with her, or that I had to install a ring doorbell to keep an eye on him when he was wfh, then that would be a good sign that the trust between us had evaporated.

Do all women think they live with men who can't be trusted not to have sex with anyone they can when they can?
Do you need to keep a eye on them when they go to the toilet in a pub? When they go to work, do their wives think they are secretly going to an orgy?
What a sad world!

It’s a bit more nuanced than that- surely you know that.

It’s not like you go to bed one day trusting 💯 and wake up the next day feeling completely betrayed.

You can trust your partner and still then notice something subtle which makes you feel a bit off and question it - exactly what the OP is doing right now.

Or, Are you suggesting the moment something feels a bit off people immediately start divorce proceedings because no one is going to leave a marriage on a whim like that are they? You notice something, it feels a bit wrong so you talk to your partner about it which is exactly what OP is doing.

Sometimes it’s completely innocent, sometimes it’s not, sometimes someone might be crossing a boundary and not even realising it etc. It’s healthy in a marriage to discuss these things.

I think helpful in terms of occasionally knocking for one off things is being neighbourly. My husband does our elderly neighbour's garden when he is doing ours. I have sorted her bin whenever it's blown over and brought them back in, (until the council started doing it). She knocks in an emergency and we do what we can. She's asked us to be present when someone was trying to sell her something. All those things are fine and agreeable to me, because she is not capable and it's occasional. I would not be happy being interrupted daily while wfh, to do things that I have done myself! There are better coping strategies she could learn and save these knocks for better occaional things like putting together something.

@CurlewKate , Apart from anything else-do people really live in relationships with men who can’t be trusted in the company of another woman? Even if the woman is -and there is no suggestion than the NDN is-a bit flirtatious? Isn’t that exhausting?

Some do. Many of us completely trust our partners and see nothing at all wrong having friends of the opposite sex.
Until we find out that it had become an emotional affair or physical affair in plain sight.

You could say that the 'helpless single mum' routine is flirting.
Most mums, single or not, get on with it, not rely on a married male neighbour for tasks and lifts.

The complaining about OP to her DP was way past overstepping the boundaries.
Why can't you see the red flags?

Am I missing something? She’s just asking for her getting her pram up and down the stairs… and asked twice for a lift, which she didn’t get.

You seriously have a major issue with your husband carrying a pram up and down stairs to help out a single mother?

Of course, if there is reason to suspect, then that's different. But the point at which I no longer trusted my DH would be the point at which I ended the relationship.

It’s not that I even feel annoyed because I had to do it myself in the same sort of situation (and at one point with a baby and toddler) as at the time I just got on with it and although it was hard some days I just did it and I wasn’t stressing about it beforehand or having to take time to recover afterwards like ndn seems to need, I was wondering at first was it some kind of anxiety / overwhelm or because of a c section (is lifting not allowed for a set time afterwards?) but then she hasn’t come across as particularly vulnerable just more entitled (unless that’s some kind of defensive thing). Dh has been slightly quiet and not his usual self I feel like I’m picking up on an atmosphere or that’s he’s worrying about her ?

I know some posters have said did he move her there / is the baby actually his !! but 100% it can’t be the case as these are council flats so unless he could influence the allocation process it wouldn’t be possible !!!

@MrsBennetsPoorNervesAreBack , I've trusted every partner/husband I have. Fat lot of good that did.

It’s not that. Most people DO trust their partners, until when something feels wrong, then they have to trust themselves.

I think it's clear from the thread that a lot of women don't trust their husbands/partners.

I find it hard to get my head around, personally. If I didn't trust DH, I wouldn't want to be with him.

Apart from anything else-do people really live in relationships with men who can’t be trusted in the company of another woman? Even if the woman is -and there is no suggestion than the NDN is-a bit flirtatious? Isn’t that exhausting?