i am catching a train from HK to guangzhou and then flying out of Guangzhou to go to Calcutta.

as my arrival flight is not coming into china but instead fly's into HK, even though my stay in Guangzhou is just two days, does this mean i still need a visa and cannot use the 72 hour free transit option?

i am travelling next week so i am cutting it fine to get a visa in the UK sorted before i fly!