Hi Michael,
I have the same model which is now 35 years old and, like yours, still working fine.
I do my own maintenance in terms of cleaning the build-up of carbon from the internal apertures of the water jacket by using a fairly wide bottle brush from the top and sides, as described in the installation manual ( a copy of which I still have). Just make sure that the burner tray is covered to prevent ingress of carbon.
I have recently replaced a couple of radiants and the thermocouple, which I find needs replacing every few years, which I sourced from:
Gas Appliance Spares
412-414 Watling Street Road
Ribbleton
Preston
PR2 6UA
Tel: 01772 702755
Hope this has been of help.
Gerry, October 2007