Automatic master/slave power switch...

[Nick]

...but the S202S02 would probably have done the job if I'd discovered it (was it around in 2004, or did I just fail to notice it?)...

It was around at the start of 2004 (just) - http://www.sharpsme.com/download/s202s02-epdf - see the rev date.

The S202S02 series seems to have been discontinued, though Farnell are still stocking them; The 16A version, S216S02, is double current (16A), the same size, in full production and is much the same cost as the S202S02at Farnell - http://uk.farnell.com/sharp/s216s02f/rel...dp/1618480

Not sure the resistor in the IR LED is necessary, but including it is no problem...

In the several years since I made these units, they've been completely reliable...

There should really be two fuses - one for the master @ 5A, (limit of the If of the BY500-600s) and one for the slave rated according to the scale of the SSR and the power strip you are using...

The BY550-600 diodes are not sold by some resellers - any reasonably good 5A or greater silicon rectifier with a Vf of around 1V should do...