On Fri, 11 Nov 2011 15:36:22 -0000, john east wrote:

Have an ordinary house smoke alarm. It's about twenty years old and it
has recently gone off in the middle of the night for two days running
and for no apparent reason.

"gone off" as in full alarm or the low battery beep? See other
posts...

Or does it probably mean that I have to replace it?

FFS a good quality smoke alarm is around a tenner. What price you or
your loved ones lives?

Smoke alarms are one of the few safety devices that really do save
lives. At 20 years old don't **** about buy a decent new one, perhaps
even get the local fire service to come and do a fire safety check.
This is free and may well highlight dangers your are not aware of.
They may even fit new smoke alarms for free, though they may only do
that if there are no existing ones.

--
Cheers
Dave.