STBXH is proving to be even more of a shit parent than I thought.
DC were with him this weekend. We were chatting this morning and they told me that at one point STBXH left the house to post a letter with my eldest DC, leaving my youngest playing. Whilst I'm uncomfortable with this I understand the postbox "isn't as far as the shops, mummy, he wouldn't let me stay at his house whilst he walked to the shops." 
They then tell me that this happened when they were staying with a relative over October half term. STBXH and eldest DC went out on a dog walk. Relative was out at the time "at a meeting"; youngest was left at home because he was poorly, waited for relative to return. Very very rural location. I have no idea (DC had JUST turned 7) how long youngest was left.
AIBU to ask whether this is even legal let alone neglectful?? I wouldn't dream of leaving a (poorly) child home alone. I just asked "what would you have done if the doorbell rang?" DC said "answered it."