Malcolm Davis wrote:
I have a two Baldor Sequence multi-duty 240v 1/4hp pond pumps (around 12
years old). I currently just operate one for economy.

It runs dry when the plastic pot filter gets blocked with pond debris
(plant and algae slime). Frequent cleaning of the filter mostly avoids
the problem but not always. The pump can run dry for many hours (such as
over night or even longer if I'm away) and obviously the pump gets
extremely hot and bothered (as do I!).

I understand that most modern pumps are fitted with protect to avoid
them running dry but I have been looking in vain for a device that I can
retro fit that can detect when the pump has run dry (or hopefully
before) and could kill the mains supply. The pump is housed in a
weatherproof structure with mains connection and isolating switch.

Please, does anyone know of such a device or how this could be achieved
economically, (without buying a new pump)?

You could attach a Thermal Switch to the pump to cut out when the pump
gets to an objectionable temperature.
See for instance http://www.thermodisc.com/productdetail.asp?ProductID=2
Bob Williams