Complete loss of coolant

Even if you dont physically see the coolant leaking, it could be a cracked cylinder head, with the coolant seaping into the fuel.

Try running the car and going to the exhaust. If you smell a "sweet" smell coming from the back, this could be the problem. Look at your oil cap, is it sludgy or greasy? Could be a cracked head or similar.

Better take it to garage to be safer rather than sorry.

The oil cap is not greasy or smudggy.

All the coolant is gone (went very rapidly), so the testing the exhaust is bit diffcult.

Was suggested that the head gasket had gone, but none of the signs of these failure are there to confirm this.

Also Suggested that cause could be a leak from water pump - how do you confirm this?

A pressure test on cooling system will show any external leaks on cooling system , hoses , rad , water pump etc .If all ok , a sniff test will tell if cyl head / gasket gone .Simple tests .