Thanks for all the advice.
I'm still really not sure why we would need a hearing test though - she was talking at 10 months, 3-4 word sentences by 18 months, and currently she understands pretty much everything said if not too conceptual - can follow complex instructions etc, but gets flummoxed by describing emotions, dual meaning words - that sort of thing. Her factual recall is excellent and after having a new book from the library and I've read it to her 3-5 times she can then recall verbatim most of it - eg the owl and the pussy cat, or Julia Donaldson books. She has definitely heard it. Language is not an issue!
She "knows" exactly what steps she needs to do to use the potty, dress, etc - just won't do them!
A prime example of her behaviour was yesterday. We went to a venue to see Santa with some friends. A venue she has been to many many times when it's quiet. The problem was there was a bouncy castle - and she freaks out over the fan noise. Wants to leave, is screaming, covering her ears. As soon as the doors close behind us on the way back out she wants to go back in as she's been looking forward to it. I reiterate behaviour expectations and go back in.
Repeat, except someone speaks to her to try to "help". Worst thing ever! She has used up so much in dealing with the noise she can't deal with anything else. And people see this as a tantrum. But it's not!
Anyway, eventually we get in, are able to stay for an hour or so with a few breaks in the hall from the fan noise, we go home, all good.
I have also left a message to chase up the HV today and will wait for them to get back to me before I go down the GP route.