I had not practiced changing inner tubes, as suggested by Jason, so I had to learn on the fly (or on the edge of the carriage lane). Anyway I managed it. The most complex part was getting the tire off at the start, and then, at the end, getting the pump to engage so I could inflate the new one.
So, although I was out this morning for two hours, between the tire stop, rest/drink stops and traffic light stops, I really only spent an hour or so actually moving.
My trusty new Garmin Edge 305 did give me all the vital stats, including a max heart rate of 159 bpm, average (moving) speed of 9.2 mph and total ascent/descent of 1000 feet. The bottom to top run up Heartbreak Hill is about 150 feet.