Twenty Pounds was a lot of money during the era of the Old Note issues.
The notes are rare, and seldom offered.
When I started collecting these around 20 years ago, few notes of this denomination had been recorded, other than specimens and proofs.
They do turn up though, and several collections have been sold over the past decade which had £20 notes in them.
So far, £20 notes of the following banks have been seen:
Bank of Ireland
National
Ulster
Belfast Banking Company
Provincial Bank
I have not yet seen any issued £20 of the Northern Bank.
Pictures to follow.