I have three smoke detectors from Screwfix (product 19792 MAINS SMOKE ALAM
PHOTO OPTICAL) all linked together.

They worked fine for 6 months - then they would just randomly go off and
sound for about 15 seconds

I don't think you can tell which one triggers the alarm, but I disconnected
one - and left it for a few weeks - nothing happened so I assumed it was
faulty and brought another one.

Two weeks later and the middle of the night the alarm goes off for about 10
seconds

Then another few days pass and they go off again

Anyone had similar problems?
Anyone know how to fix them?
Where should I start looking / how can I diagnose what the problem is with
these?

Mark