Setting up new QB for MTD, still learning.Can I manually enter random non-repeating, non-standard VAT amounts on a sales invoice ? eg £100 + £9.30 VAT

You'd have to overtype the VAT amount, using a VAT Unlock process. Set the transaction to "Amounts are Exclusive of VAT"

Split the bill into the amount, plus the VAT. Type in the amount before VAT, then on the next line, to the same account, put a penny in the amount box, with the VAT Code 20%.

Then, on the next line, put a negative penny in the amount box, with the same account as the rest of the transaction, put the VAT as Exempt (0%) - You'll then be able to type your specific VAT in the VAT box in the bottom right.