Thanks for the input fellas.
I read the CLR bottle and it said not to use on open filaments so I
went back to the old citric acid solution (ph 2) and set it boiling
for a couple of hours.
This seemed to soften the deposit and the brass brush on the drill
worked a bit better. A couple of hours in front of the tv with the
drill and it looks good as new - except for all the fine scratches!.
As a test i timed the boiling speed. It got 28 litres of water from
20c to 80c in 55 minutes. A bit of insulation should help that.