Alarm started beeping every few seconds yesterday so I changed the batteries.
It has started again just now.
I brought it in from the kitchen (it was on the wall outside the boiler cupboard) into the sitting room, shutting the three doors between the boiler and here, but it still beeps.
Is this a genuine cause for alarm do you think?
Any other thoughts?
Some alarms have reset buttons. Remove the batteries for 3 minutes then re-insert. Take alarm outside and try there.
What Make/Model and how old is it?
Rather than take a chance then I'd suggest you change it.