Thanks for all your replies!
My dp was actually the one who arranged things with MIL. i.e. she rang him up said they were going for a meal for her birthday on set day. would it be ok? and that she would get back to us with the place. He asked me if it was ok - I said fine, not doing anything that day. Then a few days before she told him the name of the restaurant. I never actualy spoke to her so couldn't ask re paying. My dp was dead set on going (he's a bit of a mummy's boy) so there's no way he wouldn't have gone anyway.
we sent her a nice bunch of interflora on the day (a few days earlier) by way of a present. The meal was definitely billed as a sort of "party" - and was only very close relatives - their 4 kids +3 partners; 2 siblings of the MIL and partners; 2 nephews; the granny; our dc.
PIL not badly off and could have afforded it - always boasting about privately educating their kids etc.
Also we have paid for meals for them come to think of it, if they visited sometimes we would take them out and always pay, also takeaways they never even offered anything.
Together we spent £70 on meal, £20 on petrol and around £40 (not sure as dp ordered them) on flowers. ok flowers we didn't need to buy just feel that the £90 on petrol and meal we didn't really have a choice.
Re my friends - ok probably I was BU then - I just should have refused. Just she was bigging it up so much and making such a fuss I didn't feel able to. I only mentioned it here as was fed up re the MIL meal and remembered I'd been stung over another birthday meal recently..