Spen - There have been times when I've run my LSR below 70% maxHR and the pace has still been well within the "running" category. But these have been at times when my mileage has been high and I've been in full blown Hadd mode.
GazOC - nice subLT session. Your 5K PB is very close to mine, as is your sub-LT pace (agree, your 10K and HM times will get a good kicking next time your rcae them!). After warm up, I'd start the next sub-LT session at 7:15/m for the first 2-3 miles (ignore HR, it will be below 160 bpm) and only after that stage stick to the 160 limit. Those first few fast miles make the session harder than it needs to be. I was doing two sub-LT sessions per week and it was indeed very hard! Hadd does suggest that you "build up the distance you can run for, over time, to 10 miles", so another approach is to start with two 6M sub-LT sessiosn per week, rather than a single 10M session.
Keir - I think there's more toothpaste in your tube. If you can run faster at low HRs, then that should convert to faster paces at higher HRs.
VTrunner - great to see you come back so strongly after your woes!