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Just opened up my fob which is also for ph2 172 and it's different to the above pic and is a cr 1220 as per gusy's post. Just open the thing up and check what is in yours. Flat bladed Screwdriver from the back like @Gaz says is straight forward enough
Of course the PO may have replaced the battery before I got the car and put the wrong one in.... it works though so...