Recently I've been doing some voluntary work at a local church, and
I've come across a strange arrangement with their gas heaters.

The appliances in question are large wall-mounted convectors with
integral thermostats. On the bottom of the heaters there is an
electrical connection, taken from specially installed
timeswitch-controlled circuits (so not just tapped off the socket
circuit). This feeds only a small heating element (actually a small
enclosed wirewound resistor of a few watts rating), attached to the
capillary sensor tube of the thermostat. There are no electrical
controls on the heaters themselves, in fact the resistor and an
associated terminal block seem to have been fitted on installation
rather than in manufacture.

So what's going on here?