Day trip to Guangzhou - second time!

Hi there,
I asked a question yesterday and received an email saying I had received an answer - but now I cannot find my question (or the answer) anywhere! The link in the email just takes me to this page where my question does not appear now.

My question is: I want to travel to Guangzhou from Hong Kong and back in one day on the train. Will i need a visa for this one day in to mainland China? I am a UK citizen.
Thanks