HMRC are always a little suspicious of businesses that claim a VAT repayment in their first VAT return, or a large refund in a later return. In such cases it will write to you asking for supporting evidence, such as copies of invoices, to be provided within 30 days.
You can now provide the requested information online, through a new portal HMRC has set up, especially for this purpose.
You need to quote the reference given in HMRC’s letter which will be in the format: CFSS-1234567, and supply other details about your business, including the five highest-value purchase invoices relevant to the period.
HMRC should reply within seven working days, and this should speed up the repayment to you.