As title - Jim Potter and I had a problem when installing his heater, his inline fuel pump inlet spigot was 12mm with 11mm hose from the tank and of course only had Tee pieces up to 10mm. (bigger ones on order! )
Is the tank outlet spigot 12mm? - I didn't get chance to measure his as we were in a rush to get it finished. It is just odd as the fuel line drops down to more normal size rigid pipe after the filter.