you need the Glysantin G33 coolant (orange jobby) ; don't just top up with water, if the tanks empty, it's empty, regardless of weather the tank is stained orange. The antifreeze also acts as a corosion inhibitor. For the sake of £3 op, buy a new cap to your expansion cap, as it's not uncommon for them to wear and release when the pressure builds up (its what they're designed to do afterall) and as everyone above has said, keep an eye on coolant from now on..