EML on and getting P0128 error. Local mechanic diagnosed coolant thermostat issue (temperature gauge swinging, reading temp of 80C but measured temperature 40). Replaced thermostat (bit of a pain as it's not easy to access) but unfortunately EML came up again.
Any ideas?

I had a similar problem, make sure the screen shown in photo of my thermostat below is NOT installed, it caused my B250 to continue to overheat until the screen was uninstalled.
see also https://www.benzworld.org/threads/b2...-saga.3102939/

Thanks. Is it that diaphragm looking bit on the left?

yes

Is it like a filter or is not permeable?

yes, like a filter.
I don't know why it was included in the thermostat package, maybe the screen filter is more appropriate for a different model of engine.
The MB dealer who supplied the thermostat part to my independent MB mechanic, supplied a free replacement after they found out the screen had not been removed before installing the thermostat, the MB dealer considered it their fault.
Good luck!