Starting problem Fiat Dicato 2.5 non turbo diesel camper van?

Hi,
Im having problems starting my van after having it overhauled recently. The reconditioned starter motor dosnt appear to be recieving sufficient power to turn the engine over. Ive had the battery checked out ok,and have by passed the ignition switch and checked the earth connections without success. There is a clicking noise from the starter solinoid and the battery (-) terminal becomes very hot after a couple of attempts to start the engine. The starter motor is not jammed and the flywheel is rotating freely.

Any advice,other than to scrap the vehicle, would be greatly appreciated.

Best Regards Jim Clements