Baby sitting

Looks like a break down in communication. You obviously both had different expectations of what helping out entailed. You need to find alternative cover should the situation happen again so that you can accept shifts with some confidence. He didn't come to any harm which is the main thing, not that there is any reason that he should. I would just avoid the neighbour till you have calmed down, as you don't want a falling out.

When you can, I would start with the “positive”
He was fine
Thank you for the burger, another time I will leave tea
But
A 10 year old cannot (legally) be left alone in a flat -does she realise?
I will leave you something, or help yourself to the fridge and my TV, tea, coffee etc but do not leave him in the flat on his own.
(Although it is not ideal, a 10 year old can put himself to bed, he may have felt very grown up)

Thank you for being kind, Maw and kitty.
I'm able to leave him for short periods, because I have to, sometimes, but I can't let him spend the whole evening on his own.

He just needs someone to sit in mine, to listen to him read and so on, and I literally don't have anyone to do it.

Don't cry MissA. I would feel the same a nd thwarted into the bargain. I can't help more than that I'm afraid but do understand!

I don't feel I can instruct her, as its a favour, (though obviously I would 'treat' her) but I'm a bit aghast that she could sit watching tv in her own flat and leave him down here on his own until that time.

She has already knocked this morning as usual (on her way to and from the shops) but I have ignored her, as I don't trust myself not to say something awful.

Dont cry - you and your grandson have survived! He may have found it all a bit of an adventure.
I agree your neighbour sounds (more than) a bit “odd” and might have reservations about depending on her. However in her day (and ours when we were children) we didn’t set quite so much store by childcare safety - remember all the latchkey kids?
So if you absolutely need her again you would have to give very clear instructions about how she does it.
But you have so much on your plate, - breathe