To access the timer on here you need to remove the hob top, Lid and front control panel. To do this its as below. This is taken from the service manual.
1. Remove the pan supports, burner caps and burner rings.
2. lift the lid and remove the 2 black screws and the 2 silver screws with the black spacers from the rear of the hob assembly.
3. Remove the 8 screws and washers holding the burners onto hot plate top.
4. remove the 6 knobs, springs and bezels.
5. Remove the 6 timer buttons.
6. Open the top oven door and remove the 3 black screws from under the control panel.
7. Lift the facia panel clear.
8. Remove he 2 black screws from the front left hand and the front right hand positions of the hob.
9. Lift the hob clear from the cooker.
Once that's off you can then access the rear of the control mounting panel. Squeeze in the 2 plastic clips left and right side of the timer and then push the timer forwards through the mounting panel.
Then you can swap the timer over from there and refit it all in reverse order.
All this is from the service manual so that should do the trick for you.