Thank you everybody for all your responses, which have certainly provided more food for thought. I have been a bit trapped in a cycle of just going round in a loop with my own thoughts so to hear others experiences and opinions is very helpful. But I would like to respond to some of the points raised. Sorry for not name checking.
I should say that when I refer to 'boys' things and 'girls' things it was just to illustrate the situation. I am one of those parents annoyed at the genderisation (is that a word?!) of toys and clothes.
I have never told my child she can be a boy. The way I have responded to her is that she is a girl (has all the girl's bits) but likes things boys things, and that is totally fine. I have never called her 'he', 'son', 'boy' etc... I have used 'tomboy' and given a definition for that.
My husband and I have always tried to provide strong positive role models both female and male for her. We have actively talked about female soldiers, athletes, helicopter pilots etc... My husband and I mix up the activities associated with traditionally being a husband/father and wife/mother (splitting of cooking/cleaning/childcare/gardening/DIY etc...). So at home there is a real mix. The toys provided to our children have always been a mixed bunch based on the interests of the child and some backlash of us against the annoying stereotyping of toys for 'boys' and toys for 'girls'.
The psychologist is involved by the school, not me and purely in a role of providing advice to the staff as to how to best handle the situation that has arisen amongst my daughter's class. But it does show that this is noticeable outside the family (not in my head).
So, all that taken into account, I am just posting because I am worrying about the future. I hope, like some of you have mentioned, that it is just a phase. That really would be the easiest most straightforward thing.
Thank you again for being my sounding board.