Thank you for your comments, lots to geek out on!
It was a bit of a shame the doctor didn’t take a reading around 145-155 bpm to be honest. He did show me the graph, but it was the computer that had joined the dots. Looking at the graph, the inflection point when the lactate does start rising is 145bpm and 2mmol/l.
I started thinking that maybe it was my conception of laboured breathing that was the problem, so I tested it out today on my run. After a good warm up I was easily breathing with my nose, and properly filling my lungs at 155bpm. I’ve obviosly been doing this for months now… which means that I’ve been training over my Aet…