SOLVED: TD5 starting probs (at whits end)

Sorry the solenoid trick did not work, although I suspect you have prevented a failure in the not too distant future.
A voltage drop of that magnitude will prevent starting as the ECU needs more than that to allow the injectors to operate. Find what is causing that current drain and I think you will have solved your problem. The starter motor is the obvious candidate.
Well done in keeping your head and approaching the fault logically and methodically. From experience I know how easy it is let frustration get the upper hand. Keep the faith brother.

Most likely the voltage drop is causing the relay to click then

In the end, a brand new starter motor did the trick.
I've come to the conclusion that the worn solenoid contacts must have been playing their part, plus the injector seals.
But may be all this cranking just saw off the old starter causing the massive voltage drop and poor RPM on cranking.
So a new starter fitter today and Touch Wood, it's started every time so far.
The flickering fuel gauge will have to remain unexplained. .
Thank you SO much to everyone who's offered their help and advice.

Fingers crossed, thanks for updating us

MrWorf said:

In the end, a brand new starter motor did the trick.
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Click to expand...

A good result then and I echo the thanks on updating. So many post their woes and we never get the end result.