Some good ideas there, thanks everyone. Re using bleach: the body of the spindle is aluminium so there is a risk there, though Wikipedia says it'a not corrosive at 200ppm hence the limit of concentration there. At the moment I'm still using plain old distilled water with no signs of an ominous green tinge.

Re the use of switches, thermostats etc for turning pumps on and of: my 240v pond pump is quieter than the VFD fan so I simply have it on the same mains supply switch. If the VFD is on, the pump is on. After a long cut yesterday for levelling the machine bed I found the tool and spindle shaft quite warm. I turned the pump off and the lower body of the spindle warmed up noticably so I turned the pump back on and it cooled almost immediately. This makes me more certain (as I said in the other recent thread about switching pumps on and off) that leaving the cooling on continuously during a working session will help cool the spindle faster after one cut and so reduce the starting temperature for the next. And it's simpler to implement. KISS!

Kit