one at the front, one at the back.

Firstly, the back, if it has been raining (which is a lot in Manchester) when I open the boot the rainwater dribbles off the bottom lip of the boot lid when it's fully open and either into the boot or down my neck. Could I be missing a seal/drain hole or do 'they all do that'?

At the front, compared to my X308 the X350 takes an age to warm up (on the Temp gauge) and get heat into the cabin. I know the gauge on the X308 was a "feel good" meter and didn't accurately measure water temp, but it did produce heat in the cabin a lot quicker than the X350.
I was thinking maybe a new thermostat unless there's a more common cooling system issue?

Managed to repair the o/s rear map/courtesy light which had been pushed into the headlining and removed the rear power point blank which seemed to have been glued in with chocolate, very similar to the rear armrest which had been glued shut with juice.

 

With my OBD scanner in live graph mode, I can confirm that the Temp Gauge is fairly accurate on the X350, moving off the bottom peg at 50C, moving out of the blue at 60C then moving in 10C increments for each marker dash. Sitting halfway at 90C.

Even with it warmed to 90C the airflow at HI is quite tepid, but on a longer run it does get hot. I guess a flush and coolant change (with a good bleed) is on the horizon this weekend, will change the 'stat too to remove that from the equation.

Still dribbles from the LH side of the boot into the boot when it's open.

 

The boot lid dripping isn't a bug it's a feature, (as Microsoft would say !)

Poor heat in the cabin is a symptom of a blocked heater matrix. Flushing is rarely successful. I had mine replaced at a main agent of all places, and they did an excellent job albeit it cost a fair sum. Now the heat comes through very quickly after starting the car and driving off. Last November we were still cold after driving 30 miles !

Replacing the heater matrix is not as big a job as you might think. First the glove box is removed, then the heater fan unit, which leaves the matrix exposed to be removed. Of course the coolant has first to be drained and one or two parts linked to the matrix are replaced to ensure water tightness, but that's about it.

 

Normally when the matrix blocks it blocks partially and there is hot air on the pass. side and cold on the driver's side.

There is an auxilliary Bosch electric pump in the cooling system located near the radiator.

I would consider replacing or repairing this first.

If you replace the thermostat alone take photos because it apparently consists of a number of bits and it's not obvious

how they all go back together. We normally recommend that the whole plastic tree is replaced since it can break up inside

and reduce flow.You may be able to buy it with the thermostat fitted.

Note that I have never done any of these repairs myself (yet!) but I have been reading this forum for a long time. lol