Using you Mk 1 human eyes by looking under the bonnet and under the car you should be able to spot where the leak is. It could be something as simple as a hose costing pence, it could be the water pump which will cost several hundred since it normally involes removing the cam belt to acsess it or it could be a cracked block which would require a new engine.

Without seeing the car no one on here can advise where its leaking from.

So look for yourself or trust a garage.

If you can see it dripping it seems it should be possible to try trace the drip back. Look for traces of anti freeze (pink/blue) around engine block/compatrment, pipes. If it's leaking out there should be at least traces of this somewhere. You could also check carpets around the heater vents in the foot wells of your car, if they are damp it's probably heater valve or core, though doubt that would explain coolant under car, still might as well check when your at it!

Unfortunately your description is a little contradictory. When you first noticed the puddle your car was parked up, but then you say its only doing it when engine is running.

Actually when you say "I topped it up and watched for any leaking and non could be seen so I drove home. I checked the car again after stopping and the coolant was dripping and the reserve tank was about half full "...the tank should only be half full at most to begin with. If you've been filling it up to the top, as the engine heats and the gases create pressure in the cooling system, it'll blow the excess coolant out the pressure hose. Are you sure you just aren't over filling it to begin with?