The aforementioned field tests for pressure should tell you if it’s faulty or not. Unfortunately it will need the system to be de-gassed to replace. Some cars the sensor screws onto a woods type valve (VAG group), so it can be unscrewed and replaced without de-gassing, but not Mercedes unfortunately
Got some sad news. I removed the cap on the low pressure side, assuming that's the larger pipe? Unfortunately its empty, no gas behind the schrader, so there is definitely a leak somewhere?
The condenser panel is out of sight, I reckon i need the bumper and slam panel out for inspection and access.
If it's not raining tomorrow, Im tempted to take it apart and have a look for any visual traces of dye or damage?
Sad news
Condenser is the most common failure point by far. A combination of road debris thrown up and build up of leaves in the bottom corners, they either get punctured or gently rot. Look for damp, dark areas where the system oil has leaked out.
Just an update, had a bit of free time today so took the bumper out for inspection.
Jobsworth was correct, the condenser panel looks like it's seen better days. The panel is very wet and oily at the lowest exposed section. I couldn't make out where it had been punctured but it definitely needs to be replaced. Must have been a stone or something that went through the air vents and knocked a hole on the panel.
Next step is to place an order for a brand new condenser panel. I have no idea how it's assembled together in the pack? I reckon the slam panel will need to be removed.
I wonder if I could get away from removing the radiator. Ideally would be nice not to empty the coolant, that's extra work.