My old donkey used to start okay, even in winter, when two of the glow plugs weren't working, so it seems that the glow plugs only need to be "more or less" present and correct for the car to start.
If your beast is a bit lazy to start then one thing to check is the fuel pump pressure... since apart from a bit of heat, the only other thing the JTD needs in order to start is a spot of fuel and a lot of compression.
But.... if you've had a blocked up exhaust/cat' then I'm wondering if that's hinting that you have dirty, inefficient or worn injectors? If so, then obviously that's not "optimal" .. so it won't help when you're trying to start the car when cold/been sitting around for a while.
If you haven't changed the fuel filter for ages, then it's worth getting one of those. I ran one for about 40,000 miles before I found out they're supposed to be changed every 12,000

but I didn't have any problems. On the other hand, if yours is dirty/blocked then it might be impeding the fuel flow.
Next I would get the fuel pressure (in the injector rail) tested, since your car starts more easily when it's been used in the last few hours.. which suggests that the fuel pressure is falling away and that takes time (a lot of cranking) to build back up again.
While you're at the fuel injection place ^^ they can also check your injectors. If you've had a clogged/sooty exhaust, then that might have been caused by injectors that are dirty, clogged or blocked. If they're not giving the engine a fine enough spray of fuel, then it will be more difficult to start and not all the fuel will be burnt (hence clogged/blocked exhaust... possibly).
While you're thinking about it/saving up/waiting to be booked in.. get some Cataclean for diesel engines from Halfords and stick it into a 1/4 tank of fuel and then drive it around using more revs and revs variation than normal. This will clean out a lot of soot and varnish from the fuel and injection system, so it's worth a try if the injectors just need a small bit of cleaning.
More seriously blocked injectors will have to come out and be rigged up on ultrasonic cleaning gear... but that's a different topic.
Ralf S.