The link was for battery powered alarms not interconnected alarms.
That is what I assumed Anderw had fitted.

Sorry, my mistake. I had galloped through from the OP and failed to
spot Andrew's reference to standard battery detectors.

Now the module at the top of the page may work with interconnected
smoke alarms. When I tested the output on an interconnecting wire it
went from 0V to 9V DC when the alarm was activated. The link below
seems to confirm this.

Thanks again. I think I might nevertheless try i.d.c. one of the S300
you originally pointed to as the Kidde alarms work on battery as well as
mains and it would be a neat way of making the connection. And thanks
too to you and Andrew for pointing me to things I never knew existed .
--
Robin
PM may be sent to rbw0{at}hotmail{dot}com