Problem with Mains wired smoke alarm

Hi, hope someone can help with this one: we have 2 smoke alarms wired into the mains fitted as house was built 6 years ago. A few weeks ago one upstairs began chirping and we figured the backupbattery needed changing so turned off trip switch to both, removed battery a standard non rechargable 9V job and replaced this with a new duracell which was one of the types listed on the back of alarm. To access the battery compartment we had to unplug a mini plug from the "socket" from the mains connection. (Not a proper socket, just a connection point thingy) and we reconnected this.

we thought this was it but Once the trip switch was back on both alarms go off, not just beeping or chirping but full scale alarm mode. Pressing the test button or holding the test button doesn't work, we have even tried doing this on both at the same time but it makes no difference. we have also left them ring for ten minutes to see does it cut out after that period but no. The only thing that works is turning off the mains power again. we have left it like this for the last two nights but I would rather sort it out properly than just leave it run on battery only.

Does anyone have any other ideas I can try? I have searched for posts but didn't find one on problem after replacing the battery!

thanks

If you remove the one in which you replaced the battery from its base will the other one work as normal?

I have previously found that replacing the pp9 battery in this model smokie rarely works(even with the recommended duracell or eveready) I usually replace them completely with one having a lithium cell such as EI 161. ( the complete unit has a 10 year life)

yes if the other unit is the only one connected to the mains then there isnt a problem with that one. I did see a sticker on the back saying "alarm good until 2012" or something like that. If I am trying the option of replacing the unit, are the connectors usually universal, so that I can buy a new one and use the existing connection, or is it soemthing I will have to call in an electrician to do?