alcot33uk

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