You're right, if any VAT document has already been accounted for within your VAT return, it is not possible to remove or edit the numbers.
Ordinarily I'd suggest entering a credit note, with the same numbers and setting it against the invoice but, for some reason the credit note page doesn't allow you to enter negative numbers, ie the 2nd line, so you end up with a doubled up credit note for £5,400! (Support this is a defect).
The alternative is to write it off but, for some reason, I don't think CB will allow you to do this, although you can try.
So the only thing I can suggest to correct the VAT on the next return is to enter a Journal (under tools). Debit VAT control £450 and credit Revenue £450 ticking the sales VAT box next to each line. This won't get rid of the invoice itself but will cancel the negative Revenue and VAT liability d=created by it.
By the way, you can enter VAT only invoices with one line; put the £450 in the Gross box, change the VAT to Manual and enter £450 in there too.