Had a moment of clarity with this one.
Have set up about 20 miscellaneous items with different prices. They are not added to the POS screen, just the database. For the barcode I have entered the amount.
So, from the POS screen, when you type in '1.00' 'find product', 'Miscellaneous item £1.00' now appears on the ticket.
Not practical for lots of different amounts, but works for us.