how can i fix it and replace parts required on combi vokera20/80rsflowmatic?

Google 'vokera flue venturi' , , 'temperature cycling' etc.
The flue venturi has to be quite clean for proper air pressure switch operation. You can download the manual from www.vokera.co.uk. Check APS switch operation with a multimeter, the fan heat exchanger and flue all need cleaning if this is the trouble. If you are unsure you would be best getting it done as part of the standard maintenance by a by a vokera trained professional.

c
May 2007