The answer is in your question, "low batt", it needs replacing.
There is a mains supply to your panel, this needs to be isolated before you open the box.
Open the box, I would expect it to make a noise, enter your code to stop it.
Look at the battery to see what size it is, take these details to somewhere like Screwfix to get a new one.
Fit the new battery, leads the correct way around.
Close the panel and enter your code, turn the mains back on, that should do it.
acorn, May 2012
