Is draining coolant as simple as unscrewing the plug on the bottom of the radiator? This is what I have done but only seem to get about two litres out of it, is there a drain plug on the block?
Indeed there is, it's on the front of the block closest to the gearbox side, it's about 25-30 mm diameter and has a 17 mm hex head on it if I remember correctly
Just be careful with it when re fitting, it has quite a fine thread that could be damaged easily. So just be sure it's clean and put a small amount of lube on it
You need to remove the plug if you want to get all the old out though as the thermostat is higher than the drain bolt. Also a good time to blast the system through and get any built up sludge or deposits out