Alternator or voltage regulator

[Jonpasco]

Hi all

Hope you’re all well, w123 200 m115 engine (petrol) manual.

Went for a drive and indicators stoped working lights dim. When I parked up battery light came on and it was dead. Battery was old so put in brand new one (voltage was over 12.6) started up straight away but then the battery light was on constantly.

I checked the voltage and when engine running was giving me over 13.7v, I changed the voltage regulator (from original Bosch 0192052008) to a hella replacement one. Once fitted started all ok but battery readout varies at 12.15v when the car is running even with heater on full and lights on. Battery light is still on in dash though.

So is it a case of dodgy voltage regulator or alternator? Should I check the earth? Also a little bit of oil leak from before and it’s been pretty cold wet in the uk.

Any ideas or steps to check?

Thanks