Sorry i've only just got back to this now, had a rather crappy day today.
Again, thanks for your responses. I didn't go to school at all today as my DS had a hospital appt this morning which ended up turning into an all day affair due to dropping baby off with sitter/unexpected tests being done/waiting for specialist/picking baby up from sitter etc.
Anyway, because I didn't get to speak to the school I just did a quick phone call (mostly to tell them DS wouldn't be back at all today) but mentioned on the phone that there had been an issue in the playground and I would like to discuss it with the head ASAP.
With regards to the 'it mum', no the other parents don't hold his views (well I really hope not, but don't know any of them that well!) but given that mum appears to be very good friends with lots of other mums and I am not as, until recently, my MIL did most school drop offs as I was working and at pick up time I always have to do a mad dash home because I have an older DS on SN transport. Therefore, i've not made any "school gate friends" to speak of.
I doubt her DD would be left out of parties and things. My DS is G&T and has many of his lessons with the older kids now so spends little time with his peers, so I don't think he'd be at the forefront of their minds when party invites come up. Like I said it's only recently he's began to receive invites as he's getting over his shyness. I would hate for him to go back to being introverted just because I had to "stand up for my beliefs" so to speak.
Cheese I really would've loved to stay and continue the discussion (nothing I love more than a good argument 
) but I didn't want my DS hearing his foul language and racist crap, plus I have to rush home everyday for my eldest anyway.
I will definitely speak to the head tomorrow though and hopefully he will take action. There was a few staff members in the playground (a couple of dinner ladies and the SENCO who had come out to speak to another parent) so hopefully it will be assumed that one of them had overheard and made the complaint 
. I do know the head quite well as the school helped me a lot with my research when I was doing my Med and he's a lovely and reasonable bloke so hopefully he'll take some action.
Again, thanks everyone for agreeing with me in my first AIBU!