You have some catching up to do alresdy, Nigel
Cost of voting leave (link to Independent)

That is an opinion of how much we may have lost due to missed sales/growth. It is not really something which has any method of proof. It is also rather old news (April 2019) and talks about the damage being caused by uncertainty over what will happen. We now have a clear leaving date, and a Parliament in which the influence of those trying to prevent us leaving has been virtually removed.
"A week is a long time in politics", nine months even more so.

It appears that remainers rely on the "Jim Bowen school of economics": "Let's see what you could have won".

Where (after the initial referendum) we were able to compare the UK economy with the pre-referendum scare stories of remainers, we could clearly see them for what they were.