Thanks for everyone's advice on the other thread!
Allergy consultant phoned today and we discussed ds' reaction on Monday and he has decided epi-pen is the way forward along with some speedy retesting and something a bit stronger than piriton!
Feel very relieved!
He thinks the reaction wasn't the milk in the chocolate but probably something else (I had tipped a box of toys for ds to play with onto the floor whilst in the shower and there were some pine needles from last years tree - these are the main suspect now!)
I hope he's not allergic to Christmas trees 
Thanks again! I was able to keep it together and put my point across calmly thanks to your excellent information!
Stottie 