Starter problems on 1996 300TDI 90.

Hi Everyone

I have a starter problem with my 300TDI Defender and I need some advice as my knowledge on repairs is limited but I am learning!!
When I try to start the engine it tries to turn over, very slowly before then just clicking. The battery is showing 12v on multimeter so the battery seems to be ok and there doesn't appear to be any other electrical problems. All warning lights on ignition light up as expected. I have also checked the transfer box and bulk head earth points and they appear fine. When I connect jump leads to another car, after a few minutes the defender will start and run as normal.
Any ideas would be gratefully appreciated.
Thanks
Mike

2 possible reasons ...

(1)
Your battery is dying.
It may have 12V but when the starter tries to take the current it needs, the battery can't provide/handle it.
(2)
Your alternator isn't charging the battery fully.

With your multimeter....
When you are trying to start the car, what voltage is on the battery ? [Shouldn't fall below 10.5V]
When the engine is running, what voltage is on the battery ? [Should be around 14.4V]

Thanks Miktdish

You were right. I pinched the battery off my sons car, much to his displeasure, and the defender stated first time.
Thanks for your help.
Mike