Heater Issue

[kusishka]

My son was driving home from about 40 miles away but he noticed little or no heat from the heater.  On occasions a 'burst' of hot air came through but that quickly subsided.  I checked water levels the following day and they seemed low (difficult to see the pink level line!) so I topped up with red antifreeze at 50:50 mixture.

I then drove a few miles on country lanes with little heat coming out, even though I had set both controls to max.  On travelling back on a faster road I pressed the middle button and the car warmed up nicely until I got home.  Once home, I switched off the middle button and the heat output was practically non existant.

Does any one have any ideas what is wrong or is it a visit to a knowledable garage?