Update - sorry for the delay, I was spending time obviously trying to sort things out and then been at work myself.
Thank you all for your concern and advice/best wishes etc. Really has been very very helpful.
Right. This might be long - so sorry! But basically, I managed to get hold of the actual HV and told her directly my concerns. She wouldn't tell me that much as such, but did say that she was 'keeping a close eye' on things. I can only take that to mean that she also has concerns and that my SIL is definitely on her radar? MIL can't/won't confirm to me how the baby is feeding now. It wasn't a one- off - apparently SIL is just 'delighted' (her words) that the baby isn't 'demanding' (again, her words) and that she's not 'pestered' (again, HER words!) all the time for food. 
. All very worrying to me. Honestly, I think her parenting skills are zero and her mothering instincts not looking great right now. Personally, I didn't think she was emotionally ready for a baby, but I know it has absolutely nothing to do with me. :(
wantanorange Don't worry - I've never had the attitude - can't report to SS because it's family. I actually am not close in the slightest to SIL. I am far more concerned about the baby. It's just all very very awkward because MIL in denial, and just keeps saying things like 'well wait til you have your baby and you'll see it's not that easy' etc etc. MIL upset enough to involve myself and DH but then doesn't really follow up when we ask for more info - or insist she gets involved more etc. Also agree SS much more likely to offer parenting help anyway, rather than just remove a child.
Re. feeds - we also found out that SIL is making up one bottle and basically just feeding that one bottle over goodness how many feeds until it's finished - ie. not making up fresh bottles. The bottle just sits there for hours on end. MIL has apparently 'bollocked' her for that, but we don't know if she's still doing that or not.
MIL said baby is quiet, but got colour in his cheeks. MIL is not there all the time - she goes over (or was going over!) every 3 days, but has now said she's going to visit every day. I don't know what happens when it's just the parents on their own - I have a horrid feeling that even at 5 weeks the baby knows that crying is pointless and has basically jst given up. When MIL arrived yesterday, his nappy needing changing, but he wasn't fussing - SIL just said 'well I'll do it after I've watched X on the tv he'll just have to bloody wait ' 
(in about an hours time!!!) MIL apparently insisted, no NOW..so she huffed somewhat, changed the nappy, didn't wash her hands (no2!) and then shoved a dummy in his mouth.
sigh Honestly. I feel really quite stressed out about all this. I'm angry that I'm related to somewhat who's such a shit mother, and can't even do the basics for their own baby. :(
Anyway. MIL has promised to keep a closer eye on things. She's obviously on the SS radar - and I WILL definitely ring SS myself if I think that things are not progressing, and that SIL is still having such a crap attitude towards parenting. I might just add that she's not depressed or anything (no PND) she just doesn't give a toss from what we can gather.
In terms of what SN she has - I don't know...I just have worked with ppl with SN and she shows the same signs..emotionally very young (she's nearly 30 but at times can come across like a not very mature 14 yr old), she's not bright at all (left school with zero qualifications) either academically OR just to have a conversation with, has never had a job in her life (not even a paper round or similar)..she has no life ambitions, no interests/hobbies, DH never recalls her even having one friend as a child/young adult. All very sad really.
Really do hope that HV wasn't just fobbing me off - I didn't get the impression that she was, just that she wasn't able to tell me anything because of confidentiality? if that makes sense.
Am just about to ring MIL now and find out an update from today.