Now call me a sceptic, there could be a genuine reason why that VAT number is not on the invoice, or why they haven't got back to me yet. They might not be VAT registered at all and don't know how to use the software and aren't qualified as a bookkeeper, so don't know that they shouldn't be charging VAT; there may have been a change in circumstances and they are now below the VAT threshold and so would be eligible to deregister, but then they would have to ensure that they weren't charging VAT - Would you want your customers questioning why your VAT number wasn't on the invoice - I know I wouldn't.
Where would you look for your VAT number?
If you were to have a VAT visit, would you know where to find all your suppliers VAT numbers if they were charging you VAT and you were consequently reclaiming some of that VAT?
Who should have access to your VAT number?
Well obviously the boss, the accounts department, customers and suppliers, your accountant. I would also suggest the reception staff, it will save transferring the call through to the accounts department and saves the customers time, leaving the customer happy.
Where should your VAT number be displayed?
On all your Sales Invoices according to HMRC guidelines, otherwise how do you charge VAT to your customers and how do you reclaim VAT.