With the weather cooling I found out that the Jeep heater didn't heat.
I knew the AC worked fine as we were having a heat wave at the time i bought it, but didn't think about checking for warmth.
It's a complicated system and checking on line I found lots and lots of posts about an 'air blend flap actuator' failing resulting in no heat. So after removing some cowling and panels under the steering wheel, then hanging upside down with a torch in my teeth, to see behind the dash I realised the instructions were for LHD vehicles so I was looking in the wrong place.
Refit cowling and panels, move to passenger side and remove glove box. Yay and do'h. The actuator is working fine. Yay, I don't have to spend money and time changing it, do'h, because it's not the cause.
OK, go back to basics and forget internet experts. If the vehicle is getting up to temperature, albeit slowly as it's a modern diesel and air is going through the heater matrix then that matrix must be blocked internally.
Remove hoses and reverse flush, slowly at first, because I
really do not want to remove the whole dash and steering column to replace a blown matrix and yep, gunk flows out.
Left the hose running for a while till the water was flowing clean. Refitted hoses and hey presto, we have heat.
I will do a complete cooling system flush sometime later.