BOB, there are two references to that figure.
The first is 'The UK is the first European country to enter the agreement, and the government claims it will lead to a £1.8bn boost to the economy "in the long run".'
The second one states, "with official estimates suggesting it will add just £1.8bn a year to the UK economy after 10 years, representing less than 1% of UK GDP.
A report into the impact of the deal was produced in 2021 and included this table showing the impact on various measures.
https://ibb.co/RPwV4gZ
It shows an increase in GDP of £1.8bn but that is measured over a fifteen year period.
Some of the measures show an increase in imports so when adding the increases together, that is deducted and is where I got the overall £4.1 billion change from.