Thought I should update everyone on progress.
After speaking to the online chatroom representative at about 6pm last night, this morning at 10.30am I received an e-mail from the Food & Beverage Manager at the St Regis Beijing. The lady apologised for the inconvenience and stated that while it was difficult to refund the equivalent of 15% for "security reasons" (I'm not sure why), she invited me to try the Signature Afternoon Tea for free (or a meal in the Garden Lounge restaurant) and to meet with her personally as a valued SPG Gold guest.
I love afternoon tea and it is certainly worth much more than the 30RMB the discount would have given me, so I'm very pleased with the result. Looking forward to trying it out soon!