Ian White wrote:
[...]

One of the sheds or big catalogues sells a PIR light with a radio link
to a 'chime' that can be plugged in anywhere indoors to warn when the
light is triggered.

There is a choice of three radio channels, so if you decided to have two
PIRs, they could be either be operated as separate systems on different
channels, or both set to the same channel so that either PIR would ring
both chimes. (As with all low-cost "wireless" systems, the downside is
the risk of false alarms if the neighbours had similar devices
installed.)

Bought one for the relatives recently, for exactly the reasons you
describe, and they are quite happy with it. I just need to remember
where from...

Okay, problem possibly solved.

I bought a 'Jaguar' (IQ Europe Ltd.) 'battery powered PIR sensor
with portable chime alert and light adaptor'. (In B&Q, 50 quids.)

Incidentally, initial impressions: (i) I wonder how long the AA
batteries will last? (three in sensor, three in chime); (ii) I wonder
will the PIR sensor go the way floodlight sensors go when exposed to
weather Irish weather (and facing south-west), i.e. fail after
two/three years? (iii) cats; I'm now attempting to use some sort of
netting to discourage them from their 'traditional route' into my
garden; (iv) set off by the onset of bright sunlight; (v) easy to fit
and set up -- ten minutes at most -- compared to something that I'd
kludge myself.

Thanks all.

Best regards,

Jon C.