Take a look at the battery in the clock. It'll probably be an AA or AAA type. Check if the battery is one of the cheap carbon-zinc types or an alkaline. If it's an alkaline, take it out and replace it with a carbon-zinc type ( I bought a pack of ten recently in a poundshop for a quid).
I've been infuriated by a wall clock at home that has failed to keep time despite having about four new batteries installed over the past few months. I was installing new Duracell alkaline batteries every time. I then came across a thread on here that advised installing carbon-zinc batteries instead. My clock works like a dream now.
I think it's worth a try in this case merton69. The reason for this is something to do with the battery voltage supplied to the clock movement. Some clocks don't seem to like a supply from an alkaline battery.