What is the replacment battery for worcester bosch thermotechnik phase clock and how do I fit it.? 18v 2.4ah bosch battery

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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 pane of the boiler. I think I will need to remove the front panel of the boiler to gain access to it. Any help appreciated, equitable want to order the battery ready, and not take the boiler to bits until it arrives.

18v 2.4ah bosch battery