Thank you so much for your detailed reply, Siaron, and assuaging of my anguish! I didn’t mean to imply that I thought it was a relative clause because of the ‘implied that/which.’ My apologies. My more complete initial thinking step was: there is an implied that, but that that can just as well be a which, therefore there is an implied that/which. It seems my fundamental problem is believing, without exception, the statement on page 43 of GK’s Intermediate Grammar: “If you are dealing with a relative ‘that’, then you will find that you can replace it by ‘which’ and still have the sentence make sense.” Thus, I excluded the sentence being a subordinate clause with ‘y’ as the appropriate answer (Unit 12 of Intermediate Grammar) simply because this sentence under the microscope made sense using which. I’m not very good at putting what I am trying to learn of grammar into conversational practice, but I don’t fret about that, and I do rather enjoy the challenge of asking myself ‘Y?’ over my early morning pot of tea!