Becks -- I wonder why your coolant is low? Could it be leaking or boiling away? Check for whitish staining where it is leaking, on hoses, where hoses connect, or round the radiator. Not just the big hoses to the rad, but also the thinner ones to the heater and carburettor (if you have one), and to the header tank. Also round the pressure and filler caps (if separate). Do you notice a trace of steam as you pull up from a run, especially on a cool day? This could be water leaking from the rad, which could be dry by the time you get the bonnet open. Any steaming drips under the engine?
Alternatively, is it running too hot (once topped up)? Faulty thermostat or water pump, blocked rad element (inside or out), blocked hose etc etc. In this case it will be leaking through the pressure cap.
A possibility which can be very tricky to find is a crack or leak above the air-space in the rad. This can leak clear steam when under load, but not water, and perhaps not on tickover at all. The pressure in the coolant is too low, so the engine boils inside, constantly losing steam.
The coolant could of course just have run low, but it needs checking out, perhaps with a pressure test.