What platform for the 1229 to pensance from preston station?

Look at the departure board when you arrive at the station, making sure you realise your destination is spelt 'Penzance'. That will give you the platform number. There is no point in asking so far in advance on this board for, whilst trains tend to use the same platform from day to day, operating reasons could cause that to change at the last moment. You will travel in a red and silver 'Voyager' which has a display beside the carriage doors giving stations served and the final destination so you can check and be sure you are getting on the right train.