Cheekymonk- I have been in a similar(ish) situation with my sister.
Sis has a massive drink problem. Not sure she's technically an alcoholic as she doesn't drink everyday, but when she does, she doesn't stop until she's drunk everything in the house. As far as I know this was happening 3-4 times per week. She was (and I suspect still is) frequently paralytic whilst solely responsible for her 7 year old son. Husband, (who she is currently getting divorced from) is often away on business during the week.
Sis is also up and down like a rollercoaster, to the point that it is suspected she may be BPD.
After having tried and failed to talk sense into her & her DH on countless occasions, myself and another family member finally made the decision to report her to ss.
What they did was the following:
Contacted the school to see if there were any concerns there.
Spoke to sis and her H (via telephone) about the situation.
Fortunately (or unfortunately, depending on how you look at it), the school did not report any major concerns, and as Sis & H did not deny the problem (although they did minimize it), the case was closed. It was closed on the understanding that sis would visit GP to get help with her drinking, but infuriatingly, ss do not have the power to check that she has followed through on this promise. (and I would be gobsmacked if she has)
ss also told me that if at any point I became aware that she was in sole charge of DN whilst paralytic, the best thing to do is to make a 999 call to police, who will then do a welfare check. Could this be an option for you?
I agree with other poster that said best thing could maybe be to talk to neighbour about your concerns first of all, but if it carries on make a ss referral. You will be complicit otherwise.