Carer charging me even though I didn't employ her

[NickyNora]

My son is Autistic.
We use a Carer on a regular basis.
One of his friends from college had an outing recently.
The other child's parent organised the activity and the Carers.

We all use the same Carers.

The other child invited my son and 2 other boys to the outing.

I thought my son was a guest.
He needs almost no help socially now especially when he's comfortable with the environment and company.

Yesterday, I received an invoice for £160 from one of the Carers.

I checked with the parent of the child whose outing it was, that they have already paid the Carer.
The other parent didn't know I was being billed.

AIBU to say to the Carer, that I won't be paying her as I wasn't aware I was being billed?

I think in your place I would email/text/write to the carer so that you can find the right words.
I would say that your son was invited to the event, didn't need care and you did not even know that the carer would be attending, so she will understand why the bill is a shock and would blow your caring budget for the month for care that you didn't book.
You would not have sent your son if you'd known that it involved caring costs as you can't afford that. The booking parent wasn't expecting you to pay either, as they have paid her. Up to you, but I would ask if we could come to an agreement. If you value and rely on her input I would pay some of it in instalments.
See how she responds.
These relationships are tricky, it is not like refusing to pay for faulty goods or services bought from a shop. She was expecting to be paid for a long day, you were not expecting to have any care from her that day.

I hope you did enjoy the respite of that day at least. It sounds like your son enjoyed it.

"She was expecting to be paid for a long day, you were not expecting to have any care from her that day."

She had no right to be expect to be paid anything by the OP. She made no arrangement with her. She is a self employed carer and she will know how it works. She is trying it on.
I understand why the OP has paid because she is worried about having no support for her son in future but this person has no right to expect any payment for what is at best her own stupidity.
You wouldn't pay a tradesperson who did work that you had not booked them for.

She is taking the mick big time!! So her rate is £15 an hour but she's charging more than one person so she's effectively working more than one job at once, but then you also say your son didn't need her on this day and you didn't employ her. No, tell her you're not paying but in a reasonable way so as not to upset her. And also say that in future, if you need her, you will contact her and agree the hours and payment beforehand.

I would explain that you won’t be able to do anything else this holidays because of this unexpected expense.

It depends on the set up. Who actually took your son to the party and who was responsible for him while he was there. I agree it just needs to be sorted out.

Sorry outing not party.

It happened a year ago so I presume it’s sorted at this stage.