Talk

bit cheeky isn't it ? no doubt she thought she was missing out on something good.

You need to explain better.
Was it a public event (fete? Dog show? Sports match?) you went to?
or
Was it something that someone was hosting (say a BBQ)?

Who made the arrangements ?
Had the Nana been asked if she would babysit so the parents could have a child free afternoon? or was she just tagging along?

Your OP is all a bit unclear.

Event was D&D, (I'm a bit of a nerd), a few friends, including mummy friends, hosted by BIL. No mention of Nana being there, I have never once asked her (or anyone else) to babysit. She never asked. I don't believe she intended to play D&D, she just wanted to snuggle and play with DS.

An event hosted by BIL, so her son? And she wanted to look after your DC so you could play? Presumably it's easier to play D&D when you're not fully focused on a small child? I'm assuming there's a back story here but from your op and your update YABU

A grandmother wanting to play with and snuggle with her grandchild?! Shocking!

If your son wasn't playing, then certainly it would be easier for you to play of his grandmother was occupying him.

Unless there's a massive drip-feed/backstory en route, I really don't get why this bothers you. She wanted to spend some time with her grandchild and presumably nobody else was upset by her presence

You left the event because you were miffed that your kid was being looked after by his Nan?

How was she gatecrashing? It was her son's house and he knew she was attending. I guess if you don't like who invites you don't have to go.

Who were the friends' children going to be playing with once your DS had left?

My DS was set to play with my friend's DD, I was more looking forward to hanging out with my mummy friend and watching our kids play than the D&D. If she had asked, I would have said no. She just presumes she's needed.

Does Nana ever get a look-in elsewhere?

I don't believe she intended to play D&D, she just wanted to snuggle and play with DS

I was more looking forward to hanging out with my mummy friend and watching our kids play than the D&D

Double standards going on here.

Your son could still have played with your friend's daughter with his grandmother there, though, and you could have still hung out with your friends. I'm still not getting the issue, I'm afraid.

It wasn't at her sons house. It was at my DP's workplace, but BIL organised it. If I had known beforehand she'd be there I wouldn't have gone. I just didn't want to deal with her for once and actually get to see my friends. It was only a couple of hours and my mummy friend was going whether I was or not. I do think I was U for storming out, but this was the first time in a long time I was meant to have Nana free socialising time!

What is D & D?

Sorry, what's D & D?

What was the problem with her being there? I don't understand how it affected you very much, other than the fact that you clearly don't like the woman.

Is your 'mummy' friend different from just a normal friend?

What on earth is D&D?

YABU for saying 'mummy friend'

If you were there and your DS, and your BIL were there, then it was a family gathering more than a social event really, wasn't it? She probably thought she was being kind and helpful - I can't imagine playing D&D while keeping a vague eye on a child is very doable. Cut her some slack - sounds like a well-meaning misunderstanding more than anything else.

Dungeons and Dragons.

If she was there to see your son, how did that stop you socializing with your friends?

Dungeons & Dragons.

Yabu

I'm now saying 'mummy friend' in my head a la the Inbetweeners 'car friend'

I don't understand why her being there was such a big deal either.