Transit Custom long term starting issue - HELP

[Stevehessex02]

I have a long wheelbase diesel low roof 2014 Transit custom, had it for 6 years and looking back, can see that it’s had this ‘fault’ right from the time I bought it.
Quite simply, when the weather turns cold at the end of the year it doesn’t start! But that’s not quite it you see because I’m sure it will start given 12,13 or 14 volts but to listen to it, you would think that the starter motor is only getting 6 volts. Alternator is providing plenty of power but starter motor has been replaced by garage twice (historically) after being recovered by AutoAid 4 times over 7 years and who also couldn’t get it started, when it failed to start as the sun went down at the end of the day and left me stranded at my customers house (usually November to March).
So over the years ago changed the battery for a more hefty one but this didn’t improve the cranking speed so added a second diesel hefty one as there is enough room under the seat; didn’t use the Ford kit to join them as was to expensive but linked them together with proper battery/starter cables. Still no real improvement. So to recap the situation as of yesterday morning, turn key, wait for glow plugs to switch off, turn ignition key, engine cranks over for 3-4 seconds and normally starts but if as happened once and my fingers slip off the key, turning the key again immediately produces slower cranking and all hope of starting has gone. So yesterday I’m armed with my new purchase a NOCO GBX155 lithium starter unit with a wapping 4250 amps available apparently so connect this up, positive to the red cap on top RHS and negative to the steel hoop on the engine lower down on RHS, follow instructions and absolutely nothing, turn key, and get near dead flat cranking. Starts a 17 litre diesel lorry engine apparently but not my van, the only way to recover from this situation is (as most of these moments I haven’t got off the drive) to hook it up to my old fashioned battery charger overnight and then try again the next morning and when I have another 3-4 second window of opportunity to get it started!
Could someone please tell me what’s wrong with my van? I would be so grateful.

[Altransit]

You seem to have covered most things, but have you tried a new ignition switch, or checked the battery earth cable to the seat base bolt
What about the cables to and from the starter

[Stevehessex02]

Hello and thanks for being the only person to reply. Yes I believe I have checked them but only checking if they are loose when really should check for corrosion and making sure the connections and continuity are good!
Somebody else has mentioned this to me, hence my reply sounds knowledgeable. I will try and recheck everything everything this coming weekend
That’s all your help!
Regards Steve

[knobby1]

Hello and thanks for being the only person to reply. Regards Steve

Not everyone has a 2014 Custom.

Lord Knobrot

[fortyeight]

give the connections a good clean with emery cloth,sandpaper