VAT man Disagreement regarding VAT Invoice

My client is a second hand car dealer, and within a couple of month's of registering for VAT has been pounced on by the VAT man, who paid him a lengthy visit to basically nit-pick. He failed to come up with anything substantial, however he decided to disallow several purchase invoices my client had included in his first VAT return.

The invoices are for Warranties on the cars and comply (to the best of my knowledge) with the guidelines for an acceptable VAT invoice. There is a tax point date, the VAT registration number is shown, the VAT is split out, it says INVOICE/RECEIPT at the top and has an Invoice/Receipt number. When my client queried this with his supplier, he said that he provides exactly the same format invoice to all of his customers, most of whom are in the motor trade and they have not had any issues.

Is anyone aware of any reason why these invoices might not have been acceptable to the VAT man?

Any help would be very gratefully received.

SD