[Ei141 still beeps after battery replacement]

1st off - great forum, thanks.

I have 3 of these in my house, 1 on each floor of my 7.5 year old house..

The top floor unit started chirping about 2 days ago.

Today I replaced all batteries with Duracell (wow I did not know they were that expensive!).

The chirping did not stop.

I then disconnected the mains, pulled all covers off, vacuumed them and replaced.

Chirping did not stop.

I then set the alarm off using toast...(!). then wafted a tea towel until the alarm stopped.

Chirping did not stop.

15 minutes later there was a double chirp from each sensor (reset).

Chirping did not stop.

I disconnected the mains (green light off).

Chirping did not stop.

I then swapped them all round i.e. top floor to middle floor, middle to bottom, bottom to top.

Chirping did not stop.

It's driving me mad and I have not slept...

I want to take a hammer to the whole system except for the fact that I have 2 small children.

Any guidance gratefully accepted.

Alex