Diesel starting grief!

OK it a 326 TDS Auto UK spec. 155K mile. Initially I gave it a couple of heater runs cold and it started fine. When hot it always needed cranking a lot.
When stood it is draining back. I can see air bubbling up the clear line between the filter and the injection pump
If I turn on it gets pushed forward (lift pump running). but the bubbles back. Several hits of about 5 seconds each seems to shift it.
This has the same effect on the heaters, several hits nice and hot.
It start magic!
I have a set of new heaters ready but I am not convinced they are the problem.
Can some tell me how the start sequence runs? Should the lift pump run then stop? What governs the heat call? Can I over ride and call heater timer even when not requested? Which relay is the heater?
Sorry its a lot of questions but I don't fancy swapping any good parts and they are not cheap!
Thanks gurus! Garry Manchester UK
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