corsa c

Hey hoping someone has advice for me I've tried changing the pretensioner but the light didn't go out and the corsa onboard scan doesn't say there is anything wrong with the car but the light is still on tried checking the fuses no joy cleaning the connection under the seat, draining the system...

I have engine oil in my header tank & also thick oily sludge in the coolant pipes going from the header tank into the radiator & also into & through thermostat housing... Has been leaking alot of oil on the driveway for some time. Also starter motor is very intermittent, although its almost new...