Likewise disappointed at confusing request for payment of an invoice with “menaces”. However:
As far as I know the normal period is 30 days, and that interest can’t be charged on a civil debt. Am I right in thinking I can tell him to jog on?
All of that is unwise, certainly in England.
There will be a payment period in the T&Cs, which you will barring considerable agility and forethought be bound by. There is no magic in 30 days. Invoices can be submitted for immediate payment.
Statutory interest (i.e. payable even if the T&Cs don’t provide for interest) will run after 30 days from the delivery of the invoice I think, but if there is an interest provision in the T&Cs that will take priority. They will also be able to claim costs of recovery if you’re not careful.
I’d advise you to pay the invoice, less an amount for the cost of damage you claim they did. Send that with a statement about the damage and why you’re making a deduction. Then they actually have to engage with that. As it is, you’re just a poor payer who may turn into a bad debt. Their office has a bored and boring person who will be little inconvenienced personally by trashing your credit rating.