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I'd suggest replacing the filter if you haven't already
It may have developedan air leak on the fuel supply pipe from the tank to the engine. The engine has no separate lift pump and relies on the built in one in the injection pump. If a leak has developed on the fuel suction pipe then air entering the system will air lock the pump.Try pumping up the fuel using the hand pump in the engine bay on the filter housing, then crack off / loosen each injector pipe in turn while cranking theengine and getting an assistant to operate the primer. This will expel air from the injection side and the engine should fire up. The problem will be cured by checking the supply pipe along its length for signs of leakage and repairing any you find.
If you daub each pipe joint with heavy grease and the issue goes away then wipe the grease away one at a time to highlight the problem joint