Ignition fault, bit of a tricky one!

So, to start with, the Defender is a 1987 90 which was a 2.5l petrol but was re-engined by the previous owner with an Isuzu 2.8 TD. I've fitted a new starter motor but still have an ignition problem, basically, every now and again I turn the key and although I get the lights on the dash the starter doesn't do anything. This is only intermittent and sometimes it fires up just fine. To get it started I just use a jump lead to connect the +ve spur on the starter to the ignition switch terminal, but this is kinda tedious. Because it's a petrol been converted to diesel I'm lost as to where I should be looking. I guess the wire from the starter motor ignition switch terminal should be traced back, but I don't know if it should go straight to the ignition-switch barrel or via a relay.

Any pointers very much welcomed!!

Cheers!

just to the iignition switch, the solenoid on the starter doesnt need much power, its a relay (of sorts) anyway.
id try "hotwiring" from the back of the switch for a couple of days, its most likely the switch itself at fault, rather than the wiring IMO

what Tim said. Had a very similar issue with mine; sometimes it would start and sometimes it wouldn't. Got to a point where it was more that it wouldn't start.

When I took the ignition switch off it was cracked at the rear so although the ignition lights came on the last turn just made the bar inside the barrel twist within the switch without turning it.

New switch and it starts first time again.