Maybe I'm just naive, but I can't believe the prices of hotel rooms - or of train fares. They have soared since I last visited. For example, a hotel I stayed at about ten years ago for under 100 GBP a night is now over 200 GBP a night. A train fare that was under 40 GBP maybe five years ago is now 120 GBP. It's mind-boggling.
I much appreciate the kind advice that everyone's given. In the end, I decided to use up some frequent stay points and booked a room at the Sheraton near Heathrow airport (even though I would prefer to stay in the city). Since I plan to arrive early afternoon, I'll take a train from Heathrow so I can still spend some time in the city. Also, I shall take a National Express coach to/from Cornwall; it's one-third the cost of taking the train.
Definition of a Cornishman: "short arms, deep pockets".