Repair or replace the clock that keep stopping on my ARISTON MICRO GENUS?

We bought a house with an old combi (its a 23kw) 2 years ago and it has an old timer instead of a digital one. The problem is that it keeps on losing time so sometimes the heating comes on in the middle of the day when it's been set for 6pm. We are selling our house and dont want the new owners to have the same problem so is it simply a case of replacing the timer or is something darker at hand??
Also, we often have to keep pressing the 'reset' button as it wont ignite on turning the tap on. Is this connected??
Any help would be appreciated asap!