If a child has a very bad peanut allergy they may react even to traces of peanut protein in the air. Fortunately that is rare but if a child who has eaten nuts holds hands/ passes a book to or has any other contact with an allergic child they can give them a reaction. The reaction can be a rash or if the nuts reach their mouth it could be life threatening. So banning all nuts from packed lunches is not excessive if there is someone with a severe allergy. Peanut doesn't wash off with just water, either, so it can be on small children's hands even after they have "washed" them.
It would not do any harm now to speak to one of the parent governors. You may find that there are other allergic people at the school they don't know about. It also helps to refer to the new guidance for schools on medical needs (new because it was updated recently). Mentioning their obligations under health and safety legislation doesn't hurt either.
I know you've done alot of letters for friends in the past but maybe time to remind the others child's parent too.
I think all playgroups and primary schools should ban nuts. As children get older they can be trained to cope with their allergy better, young children shouldn't be put under that pressure.
Podmog I have found my best ally is the school nurse. They will do the epipen training an if you ask nicely they will probably let you go along. Do go if you can to make sure it takes place. Also if you have old epipens/ another trainer pen they are good for practise. The nurse ususally takes one but with the staff haven't limited time the more you have the better. Make sure you get it back though.