I know this thread is old, but I felt like an update might be useful.
Since posting this, I rested for a few weeks, and had an attempt at a hilly 600km ride, as a tester for my Mt Sufferlandria quest (referenced on this thread in a vain attempt to get more mental fortitude in going through with it). It didn’t go well - as before, 180km in, I hit a wall, physically and mentally, and gave up.
So I cancelled my ride, and did another 600 instead, after another month of taking it easy. This time managed to finish it, but discovered that it’s the Tailwind causing the problems. On the first day (320km, 5000m climbing) I slowly found myself getting more and more bloated, and eventually stopped on a hill, absolutely bonking. Within a minute, I’d hurled what felt like 10 litres of liquid, and immediately, felt vastly improved. Leaving Tailwind, and going back to alternating Lucozade & water made a minor improvement, and I managed to finish the day. On the second day (much flatter), although I was significantly pooped from the day before, I felt notably better, and finished without any problems. And without any Tailwind. (fwiw, heart rate never made it above 150, and average HR was low enough that Strava told me this effort is lower than my 3 week average. How many of you have had that message after completing 280km? 
)
So I think the HR issues above weren’t that - they’re just fatigue as @Coach.Neal.H said. The unprecedented lack of energy issues I think are related to Tailwind. It seems to sit in my stomach, and even though I’m eating other stuff as well, it does something to stop me absorbing enough energy.
Back to the drawing board on nutrition, then. Or rather, back to my previous (flawed, but workable) regime, and see if I can find something else to improve calorie intake toward the back end of long days.
HammerGel seems like a potential option.
Thanks again for all the replies & advice. It’s really useful seeing other viewpoints and experiences.