The heartburn subsided gradually, and I suspect that it was a bug of some kind rather than a health issue. The kardia readings have also settled down, but I am still taking regular readings to check.
I have been steadily increasing the exercise while maintaining an HR of less than 165. My last three rides have been hilly ones (60, 92 and 94 ft of climbing per mile) in the Peak District. I am a little slower than a couple of years ago and I am regularly spinning in gears that were once reserved for serious climbs only, but I am beginning to feel like a real cyclist again.