When I have a stiff engine to start, I do the folowing...

1) reset all the needles to default settings as described in engine manual.

2) couple of drops of after run oil in the through the glo-hole

3) tighten up glow plug, then back it out 1 turn, so it's a loose.

4) move the flywheel to the top (just turn it with a finger until you can't any more) in the OPPOSITE direction to the engine rotation (this gives the starter box a kind of 'run-up'), so it does almost a full turn before it gets to TDC again.

5) Press hard on the starter box.

I've never had to heat an engine up. Be patient and persistent, it will fire. Be very careful of fuel locking (which shouldn't happen with the plug a turn out).

When it gets running, leave the glo starter on for about 30 seconds, then tighten up the glo plug. If it stalls, repeat the entire process.