How can I fix the time on Competence B3101-4, as gets stuck at 12.00?

Re: AEG Competence B3101-4

Until recently my oven worked fine, but suddenly my oven timer has got stuck at 12.00 and makes a long high pitched buzzing noise.

If you press the timer, it stops and gets the clock going but only for 12 hours until it reaches the next 12.00.

I have tried resetting the time to the correct time, but it just reverts when it hits 12.00 to the same buzzing sound and starts flashing 12.00.

It is really frustrating - does anyone have a solution?

Many thanks