Focus mk2 coolant problem

Car was fine till 2 days ago. Noticed fan overrun on switching off,  for a couple of minutes. Checked coolant took a couple of litres, never needed more than a cup before on monthly checks. Car has done two 25 mile runs with no problems except the heater only throws out warm air, it was hot before. 

Help. Des

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UPDATE

Did 12 miles today arrived with steam and temp gauge reading too hot. Engine didn't seem overheated. Drove really well. TREAT ME LIKE AN IDIOT. I remembered having a RR Shadow and after a coolant change it kept overheating till I found out that the Shadow needs to be filled with coolant very slowly. So I refilled the focus very slowly waiting for the bubbles in the overflow tank to stop, it took much more. As a temp measure I used water. Car drove 12 miles no problem, gauge normal, no fluid loss, the heater is now blasting out hot air.

The engine runs beautifully with clean oil and no filler cap gunge, even tickover so I think it's not likely a head gasket fault.

I remind that I am 78 and homeless so my thinking is addled. 

How's this for a scenario. I forget to check coolant level and by use it gets very very low. This causes the initial problem and if I refilled the coolant properly the problem would have gone away. Does that make sense. 

Lastly I now have water instead of coolant. Can I syphon of a couple of pints and replace with 100% antifreeze or do I have to drain and replace? That assumes the problem was as per my scenario. 

Thanks for any help