Update - Could find nothing mechanically wrong with gear lift pump or drive. So rather than spend silly money I rebuilt everything and blew out every low pressure fuel line with air line including all filter housings, cooler etc. Fitted new filters and bled the system up. She started ok and died, primed and started about 4 times before she settled down and ran normally, all air removed. Now starting on the button and running fine. Just cost me £23 for 2 filters.
Given the filters were changed about 15 months ago and its done naff all mileage since I wasn’t expecting them to be the problem. However what you say about the tank and corrosion is true. I have left the tank less than a quarter full for most of the summer as I have been intending to drop it off and sort external corrosion. I am now thinking it might be better to bite the bullet on £300 for a new one. At least i’ll know the inside is clean!
Interestingly there is an article today in the Farmers Weekly reporting on a lot of agricultural engines suffering from premature filter blocking / lack of power. They are putting it down to latest biodiesel. Algal growth occurring in on farm holding tanks esp when they are in the sun. Tanks that are part empty attract water which encourages growth as does increased temperature.
Possibly what I have inadvertently encouraged??