Hey @Mr Tangerine Man
This is a pretty standard Payment on Account situation. There are a few posts on here which might help but the basics are:
(1) Enter the monthly payments as Cheques in QBO (I use Debit in place of the Cheque Number), use the Supplier Name, Account/Category = Accounts Payable with NoVAT.
If you use Bank Account Matching, use the same settings. These cheques will show up on the Supplier Register as Credits.
(2) Enter the quarterly invoices as Bills in QBO - again, use the Supplier Name but Account/Category = Utilities (or whatever) with VAT Code = S20 or R5 as appropriate.
The only thing left to do is to clear down the payments against the bills. This is just to keep things neat & to stop QBO from reporting unpaid/overdue bills.
(3) Use Pay Bills to allocate the Payments (Credits) against the Bills.
Hope this helps.