It's a long time since I had to produce VAT invoices but I know then that at the very least you had to have your VAT number on it, and it was recommended that the VAT amount was shown. That was for both private and business customers. If you look at most shop till receipts of larger stores. including food, they actually state the VAT details. Now there was a number of shops and small businesses, who were not showing those details even when over the VAT threshold, that did not show it and then Customs and Excise came down on them presumably as they thought they were keeping two sets of books, and it made it difficult for others to claim back the VAT. With everything becoming computerised it is possible to follow everything a lot more closely.
Plusnet being a large company will be VAT registered and your local VAT inspector will know that and possibly accept the invoice/receipt without a VAT number, but that may vary from area to area. However how hard is it for the VAT number to be included on all invoices/receipts? Many small businesses and other organisations do not need a business tariff and a personal one will suffice but may still be able to claim back the VAT. A lot of voluntary and small charities will fall into this category.
