The VARTA 3/V150H is one of the most popular in the Mempac range. Used in many memory back up applications, particularly for Central Heating Timers, like the Potterton.
Many of you will note that the original battery it now replaces was in a Blue Case, it was NiCd and known as 3V100R or the 3/V100DKPO. Others with a Honeywell Timer will note that their outer case was Beige and has a small locating peg. You can actually push out the 3 cells in the case and switch the outer casing with your original if this is the case.