Your choice would be either:
Train to Beaconsfield, then taxi. There is a bus serice but it runs at commuter times only.
Train to High Wycombe, then bus 31 to Penn. Buses run about every 15 minutes and stop in Castle Street, close to the Station (there will be a map at High Wycombe showing you where to get the bus). If you want to, you can buy a 'Plus Bus' add-on to your train ticket.
Thank you so much. This is very helpful.
Another old post SJHP1 - this time three years too late!