Investment Trust Companies and VAT recovery

The scenario this case addresses seems almost a law school exam question. A supplier provides a customer with services which should have been exempt from VAT but which the legislation makes subject to VAT of £100. To do so it buys in professional services from an adviser which attract VAT of £25. The supplier charges the customer and recovers VAT of £100. It deducts the £25 input VAT which it paid to the adviser and accounts to HMRC for the balance of £75. Only then does it become clear that the legislation was contrary to EU law and HMRC refunds the £75 which the supplier pays back to the customer. What happens to the £25?