Check the specific gravity and likely efficiency of your engine coolant with an Engine Coolant Tester for £1.49 at a Lidl store near you. Its a simple plastic pipette with a float graduated in 10° increments from -10°C to -40°C. They also have a Screen wash version graduated from 0°C to -30°C, probably not so critical since the screenwash reservoir gets topped up so much more frequently but there if you fancy it.