Just to conclude this thread, I ended up going with the V3 thermostat:
https://www.dmgrs.co.uk/products/r75...ant=1080328848
I was having a major service done anyway, so for peace of mind I went with this option. Fully appreciate that it's the expensive way of doing things, but I'm very happy with how it performs and I intend to keep the car long term so seeing it as an investment. The garage actually said it was less of a job than they were expecting once they got into it.