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Many thanks for your prompt reply.
Further discussion has been held with our builder who now assures us that our property was correctly zero rated at that time of building.
He also states that contracts with new buildings have been changed to allow the customer to deal direct with the house kit supplier and subsequently claim the tax refund.
Thank your for your input which which would appear to have answered our concerns.
What a shame he failed to pass on this information in the first instance.