How do I get PPH to add VAT to their service charge on my invoice?

I have added my own VAT deters and by raising a ticket, this was added to my June invoice, but it would seem that the system does not automatically do this for following invoices. I have raised another ticket a week ago, which has been "escalated", but still not resolved. I have raised a second ticket, thinking I might get a different customers services rep who might know how to do it, but now there is radio silence from PPH. How have other users got them to do this? I cannot complain formally, as they do not include details of a complaints process nor address details. I had assumed it was a legal requirement to provide a VAT invoice where VAT is being charged?