Defender 200tdi. New Starter - No Click at Solenoid so no crank

Hiya
1989 110/Defender 200tdi (factory fitted).
I replaced the Starter Motor 5 weeks ago with a new one (BritPart). No problems.
This week it just failed to turn over.
I got a push start and it worked. Turned engine on and off about 10 times no problems. Then it stopped again.
I get a click at the Starter Relay (inside fuse box above gearbox tunnel) but not at the solenoid.
I've checked the relay using a test lamp and it lights up when the ignition switch is engaged. I've also swapped it out for a different relay in case its intermittent dodgy points.
No joy... so I removed the exhaust pipe to access the solenoid. I jumped the S terminal to the B terminal to bypass ignition system. nothing happened. No click & no crank.
Removed Starter from the engine placed it on floor and connected it to Battery jumper cables. Jumped the B & S terminals and it works as expected!!!! Put it back in truck & as before... nothing!
I put a test lamp on again and it shows the switch / relay are allowing power to get to the solenoid. I connected a battery cable directly to starter attached to earth on battery... still nothing.
It is technically under warranty... but if I return it then it will work as it did on the floor...
Any ideas???? gratefully appreciated...

So I've just removed the Starter Motor again... getting quicker at it!
Rested it on top of my engine and connected the original cables to it.
(as the +ve & -ve cables don't reach) I've added in 2 jumper cables to bridge the gap... attached them to the original cables not the battery.
Turning the key and the Starter spins & shoots forward as expected.
This tells me the connections must be good... even with jumper leads in the chain.

send it back if fitted and wired correctly and jumping the solenoid wire doesnt make the solenoid at least click