What is the replacment battery for worcester bosch thermotechnik time clock and how do I fit it.?

This is the back up battery for the Worcester time clock on the front panel of the boiler. I think I will need to remove the front panel of the boiler to gain access to it. Any help appreciated, just want to order the battery ready, and not take the boiler to bits until it arrives.What is the replacment battery for worcester bosch thermotechnik time clock and how do I fit it.?If you know that model of the clock, just go to a plumbing supply store and they should have one or be able to get one or at least tell you what type of battery is needed