Fair play to you Tony its obviously a truck for life
I wish i could help with this but i have never had a fuel problem and so i haven't much looked at what goes where .
My thinking is you have a one way leak , when the pump is sucking fuel through there is no leak , but when left for long periods the weight of fuel in the line is dropping and sucking air in like a syringe drawing blood .
I would search ebay for a diesel fuel line non return valve and put it close to the pump so there is always enough fuel at the pump to fire the truck up . Once running it will self purge .
It's a stop gap fix but i would imagine it should eliminate the problem altogether until the leak worsens and starts to weep diesel . When it weeps you can find and fix it .
As i have said this is not something i have done myself so you might want to wait for others to comment before jumping at it .