sorry about the long delay in getting back to you.I've been in touch with the VAT people and they tall me that proff (!) [proof -Andy] needs to be supplied (by us) to zero rate goods being exported. They're sending me a booklet about it. We have offered to deduct VAT on orders over £50 in the past as this was what i was told was the legal requirement, but it really was a bulk order discount in effect without the need for documents and red tape. I'm inclined to keep things that way.
Sounds like a legitimate disagreement. I can't really fault anybody for not understanding precisely how tax laws work, they tend to be arcane and uninteresting.
I'm glad to see they did respond to my message and appear to be investigating the issue, even if they do ultimately decide to maintain their current policy.