Update on this issue, resolved a few weeks ago now, but forgot to post here.

So, on starting to remove the existing injector pump to fit the second hand unit, I noticed that the idle screw was barely touching the throttle arm. It was putting pressure on but only very slightly.

So I left it as was and started the engine up from cold. All good. However, after 30 minutes the same old stalling issue kicked in. Turned engine off and checked the screw. Now not touching the arm at all, 0.4 thou clearance my feeler gauge told me. As the engine/pump warmed up the arm actually moved further away from the screw, hence the stalling!

The screw itself felt loose so I readjusted the idle, with all accesories powered up, and secured the idle screw with nylocs on either side and threadlocker.

Truck has now been running perfectly for about 3 weeks.

Lesson learnt, never assume the worst and always check the basics first!

Nice one .. so tickover was so slow it just stalled ?

I think so.

As she warmed up and the arm moved away from the screw the tickover just dropped off.

When cold the tickover seemed perfectly normal to be honest but clearly the idle screw was only just keeping it on the throttle arm.

Never even crossed my mind until I saw it because I'd never have thought there would be movement in the pump/arm between hot and cold.

Also think the idle screw thread is a bit knackered so probably the idle screw has also been progressively unwinding under vibration, hence the nylocs and threadlocker.