Colin,
Have you had the flutters checked out? If not you should. Could be relatively harmless A-fib but best best to get it looked into.
There must be something in the name as it would seem we are both nutters!
I had heart scans done when I was clotted and doctors picked up the signs of it then. It wasn't subtle at the time though so even a quick check stethoscope picked it up.
As the clots healed, the rhythm improved greatly. The doctors reckon that most of the problem was caused by the hugely clotted pulmonary artery pressing on the side of my heart.
I had another heart scan done fairly recently and they were pleased with progress. Admittedly, that was done when I was resting. If they scanned me under load then I think they would spot changes.
If it starts getting worse again I will bring forward my next appointment.
For now, I will just be careful not to overdo things. I can make (say) an 85-90% effort on a short, very steep climb, but I have to leave the top 10-15% alone.