“The commissioners are under a duty to conduct a reasonable and proportionate investigation into the validity of claims for a refund and repayment and a duty to act proportionately both in respect of the investigation and in dealing with the taxable person's claims generally. See R (on the application of Deluni Mobile Ltd) v Customs and Excise Comrs[2004] EWHC 1030 (Admin) (Deluni). The duty to investigate is applicable both to the claim to the refund and repayment and to the question whether there is a right to set-off (or indeed a claim for a further payment from the taxable person). The duty embraces an obligation to keep all investigations under review. The commissioners are entitled to take a reasonable time to investigate claims prior to authorising deductions and repayments and what is a reasonable time within which to complete an investigation must depend on the particular facts: Strangewood[1987] STC 502 at 505. The availability and proper exercise of the commissioners' powers of investigation are essential to maintain the fiscal neutrality of VAT and prevent refunds being made to parties not entitled to them. The postponement of repayment of input tax pending the outcome of the investigation is, as a matter of principle and subject to questions of proportionality, entirely compatible with the Sixth Directive. Whilst the burden of proof is upon the taxable person to establish that the investigation of his unadmitted and unadjudicated claim and the failure to make a part or interim payment is unreasonable or disproportionate, the burden is on the commissioners to justify non-payment of it once the claim is admitted or established and the period of investigation of any cross-claim.”