badger7, coming late to this but I think your tactic of 'what ifs' and whether you'd be happy letting someone else do it is a good way of looking at risk taking. Sorry I took your post the wrong way.
arfur, we used to live in a terrace of tiny houses and I mean tiny (no hall - front door leading straight into 12ft X12ft sitting room etc). On summer evenings some of us used to gather outside or in each other's front rooms, with the door and windows open onto the street, and have an impromptu gathering. As the gatherings were spontaneous, it was impossible for us to organise babysitters.
My ds was a sound sleeper, we had a monitor and we used to check back every 10 minutes or so. I felt it was no big risk to join my dh in the merry throng just outside our front door. Amongst our party loving neighbours was a couple who both worked in social services. They used to tut tut (in only a half joking way) and say they wouldn't report us - this time. It used to get my back up, as they usually said this to me, not my dh, so it came across that they expected me, not him, to go back and stay inside. I felt the risk was very slim as well, since we were no further away than an imaginary garden.
In retrospect, I wish now I'd asked them to babysit for the odd half an hour so I could enjoy myself totally guilt free. Needless to say, they never offered to do this.
Sorry, this is not an answer to your question, but the social workers never said to me it was illegal to do what we were doing, if this helps.