So your solenoid "clicks" loudly (as in sounds healthy) - battery perfect, all earth issues trebled checked - for me it turned out to be the copper connector bar which bridges between the two copper terminals (once the solenoid is activated). Unbelievably this was at ninety degrees to where it should be - just the tiniest of connection was being made as there are are small lugs in the centre of the bar which just about reach/make connection between the copper bolt heads. Undo all the nuts on the solenoid cap, undo the retaining screws, de-solder the connections and the cap will slide right off and all of the above will make good sense! Re-orientated the bar, cleaned everything up and all is good! Really hope this helps someone?