Just to round this thread off have now finished the job with 1.6 litres being required to filler hole (manual states 1 litre approx!). Would have saved a lot of jacking up and down if I'd known this in advance. I have updated my handbook though.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone