Thanks for the suggestion, but I am now sorted.

*** Warning, do not take the Driver's Handbook at face value. ***

The handbook said "Drive to an Alfa Garage" so I figured that I should be
able to cover some miles and set out to go to a meeting.  On the way I
noticed that the temp was rising and pulled over, the water was low but not
too low so as I was about a mile from my destination I continued with one
eye on the temp gauge.  At my destination I discovered that the low water
was even lower and that there was a air lock in the heater, after allowing
the system to cool somewhat and refilling all seemed OK so I left it until I
was ready to return home.  The journey home was tense, with the heater on
full to dissipate heat, but the warning light was now out and all seemed OK.
Once home a check revealed that there was no further water loss.

So to recap,

1 if the handbook says you can drive to a garage, still check your coolant
first.

2 don't pay Alfa rates just to have your radiator topped up in the workshop.

3 be aware that the dashboard warning lights could mean something other than
what the handbook says.

4 Check Your Coolant MORE Often.

Thanks again,

Andy Wilson