Update: Fitted another starter motor today. Tried it first on push button switch with and ammeter connected to main starter cable.
As we wound the engine over with ignition off it was drawing 290 amps for the first four checks. However on the every fifth or sixth turn of the key the ammeter would go off the scale over 400amp limit. This is obviously the reason for the main fuse blowing ( and subsequently the starter brushes when the main fuse id bridged out).
Problem is - what is causing this massive overload as the van is very reliable and starts quickly on the turn of a key? All pulleys are running free - I also wired up a test light to the start side of the solenoid to ensure the engine wasn't running with the engine, which it is not.
We are at a loss with this one? Any advice welcome?