Head gasket was replaced 11 months ago following a 6 month long hellish search for whatever was causing my severe idle problem, it was the gasket.
The cooling fan has been running constantly since well before the gasket was changed. Fast forward to today, until last week the car was almost impossible to start when cold (20 seconds on the starter before it runs). Identified that the CTS was completely broken, gave zero signal. I have replaced the coolant temp sensor, the car now starts straight away but the fan is still running constantly.

The idle is stable and doesnt hunt but it will never idle at the normal 900rpm. It sometimes idles at 1000 rpm, you can drive 100 feet and then suddenly its idling at 1250rpm, drive 10 feet and hold the clutch in and its idling at 1500rpm, its completely impossible to work out whats going on. The brake does hiss and the idle does increase if you pump it a lot, suspected air leak in the brake servo BUT if I disconnect the vacuum line and tape up the hole on the manifold and drive around for a bit the idle issue is just as bad. It does burn a decent amount of oil, the car smells of coolant from the exhaust but I've monitored the level for the past 2 months and it hasnt even dropped 1mm so I dont understand why I can smell it. The car also feels underpowered compared to 3 years ago before everything started going wrong. I own a 1.6k coupe which only has an extra 5 horsepower and that feels like a rocket in comparison.

All in all I have replaced the fuel pump, fuel filter, HT leads, distributor cap and rotor arm, coolant temp sensor, catalytic converter and backbox, cleaned out the ECU vacuum pipe, throttle position sensor, oil and oil filter

I am genuinely completely lost on what to do, I'm just hoping to god someone has found something completely random I could have missed that would cause all this. I REALLY dont want the gasket to be gone again

Sounds like you tried and eliminated quite a few things. My 214 is an earlier model than yours, but on mine, if the fan runs continuously then the thermostatic fan switch on the radiator is probably faulty. It should be open when the car is cold.

If you are still suspicious about the head gasket, you could do a compression test and also monitor the coolant header tank for exhaust gases using an exhaust emissions analyser to sniff it.

Have you got a diagnostic tester, e.g. RoverMEMS App? This might tell you what parameter is changing when the idle sits high. Could be the Throttle Position Sensor is faulty. I think on later cars, the ECU reads the TPS to determine if the throttle is in the idle position. There is a routine for setting this up with the ECU. My early 214 uses a throttle switch to check for idle position on the accelerator.

Good luck,
Hugh