Could the glow plug loom be damaged? I understand this can happen easily when it is removed to replace the plugs. Then I guess you'd see white smoke on startup from unburned diesel. Does the outside temperature make any difference to the ease of starting, as it should if glow plugs were an issue?

Possibly an air leak in the fuel system, allowing fuel to drain back to the tank and create an airlock. Not sure if this would be an issue since, when I start my car (Leon 1.9TDI 150), I hear the (in-tank?) fuel pump kick in and prime the system before I start the engine & I guess the 130hp engine would be similar.

Poor compression caused by engine wear? Would depend on mileage and service history and probably unlikely but would make it harder to generate enough heat from compression to start a cold engine, so would probably happen on every cold start and not just some.

Starter motor on its way out, can't cracnk engine fast enough?

Other than that I've no idea. Mine starts very quickly even when the starter struggles to turn it over (recent cold weather). 659FBE is the expert on these engines here.

3000rpm in 6th? - that would be around 99-105mph then ;-) Would love to know where you can get away with that for 45 minutes. It's a private racetrack, right??