The message
from "JennyC" contains these words:

We have been woken up the last few mornings by a strange sound.........

It sounds like an alarm clock, but seems to be at different places in the
garden.

The sound is 4 high pitched tones of about one second each, followed by
about 3 seconds silence, then it starts again.

Anyone got any idea what it might be or where else to ask ??

Almost certainly a bird. You're hearing it at the same time and area
because many birds return each night to a favourite roosting spot.

Starlings, blackbirds, and some corvids are very skilled mimics and
often learn to imitate domestic sounds such as mobile phone tones,
doorbells, dog whistles etc

Janet