2015 VRS TDI - Coolant loss from expansion tank

Hi All, I know this is a topic that seems to have been covered a million times but my vRS has become a problem child.

It was losing a small amount of coolant and I would get the odd whisp of steam from the back, I diagnosed a leaking EGR Cooler (confirmed with off car pressure test) and a small water pump leak. I replaced both and thought my work here was done. However it now seems to have made losses worse, this time from the reservoir, so yesterday I did a 90 min motorway drive and by the time I got to my destination the coolant warning light was on and there was no coolant in the tank. I had concerns the tank was on its way out and popped out and bought a pattern part from NAPA (I have already ordered an OEM one but I am waiting for it to arrive), fitted the NAPA part with new cap and thought my work was complete (again).

On the way home, despite topping it up before I left, I only made it about 75mins before the warning light came on and, you guessed it, no coolant in the tank.

I have been doing some diag work at home before I get it to my mates where we can get it on a lift and so far this is what I have noted,

• No smell of combustion gasses in the coolant tank

• No Milky residue in the oil

• No oil in the coolant

• No steam from the exhaust

• Lots of coolant around the expansion tank, wheel arch liner, subframe and pooling on the ground.

• Checked for hot air in the cabin and can confirm its really very hot

I am going to pressure test the system and isolate the charge air cooler and pressure test that, but wondering if the hive mind has any other ideas about what might be going on, I have searched and read what feels like most of the forums around this topic and I struggling to get my head around what it might be.

Thanks.