I have exactly the same problem. There is a small 1.2Volt Ni-Cad battery inside that keeps all your programming safe. When this eventually dies, all the programming is lost. NiCad batteries are no longer made so a direct replacement is not possible. Sadly the effort and time involved in repairing this timer is just not worth it. Sad I know, and believe me I have tried and I am an electronics engineer who never throws anything away if I can fix it! A new (different) timer from Tesco costs about £10.
Clive Hollins, August 2009