ldv convoy 400 2.4 on 53 plate ?

I have a ldv convoy 400 2.4 53 plate

when I put the key in the barrel , turn the key , wait until immobiliser light goes out , and the heater coil light on dash go out ... I turn the key and all I get is a click ... it appears the starter don't engage at all .. the engine do not turn over .. I have to bump start bus every time I want to use it ...
changed starter 3 times this week .. same problem turn the key and it just gives 1 click nothing else
changed battery ....., checked flywheel teeth .. checked alternator .... don't know what else to do .. can any one shed light on this issue many thanks ..wayne