Steve - I'd suggest arranging to go to an allergy clinic. Some friends knew their 4 year old had severe reactions (throat closing, face swelling etc) to a few foods but they were staggered to find how many others after he went to the clinic at St Peters Hospital Chertsey.
According to this the NHS have them nationwide
http://www.nhs.uk/Livewell/Allergies...lergytest.aspx