Just because you can’t see where coolant is escaping, doesn’t mean it isn’t. Equally, a lack of milky oil in the sump does not guarantee that the problem is not a head gasket-related one. Even an external leak from a hose or fitting can be hard to track down as the hot coolant evaporates before you have time to find the witness mark or the coolant drips on the garage floor.

Your best bet is to have the car’s cooling system pressure tested. A mechanic will pressurise the system and leave it for a period of time. If the system holds pressure, that’s a good sign that there’s no leaking. If it loses pressure, you have a problem somewhere in the closed-loop cooling system. That could indicate a problem with a radiator, hose, water pump, head gasket or even a split bore or cracked cylinder head.

But there’s one other check to make that might save you a lot of time and hassle. Takle a close look at the car’s overflow tank. This is also called the expansion tank and is designed to catch coolant as it expands (through engine heat) before allowing that extra coolant to return to the engine as the car cools down at the end of the trip. If this tank is leaking (even from a tiny crack) coolant that expands into it will be lost and the cooling system will appear to be continually losing coolant.

The symptoms you have certainly suggest an engine with far too much blow-by (combustion pressure escaping past the piston rings and into the crankcase). Sometimes, the same symptoms can be caused by a crankcase ventilation system that isn’t working properly, but it’s often blow-by that’s the cause. That’s usually the result of internal engine wear which, at 290,000km and counting, is hardly out of the question.

Basically, the observations you’ve made regarding crankcase fumes are about as far as you can go without actually performing a compression and cylinder leak-down test. The good news is that these tests aren’t overly complicated and don’t take long, but they will give you a vastly more accurate idea of what’s going on inside the engine. Oil in the intercooler can also be a sign of this sort of wear, but, as you’ve been told, can also be the fault of blown turbocharger seals. Either way, it sounds like your engine is due for a freshen up in the name of reliability and clean running.

On the surface it would seem that there is a problem with the clutch, it could be that the pressure plate has collapsed or something similar. Pulling the gearbox out should enable the mechanics to find the cause of the problem.