85% HR training and what to do when getting fitter?

I'd put it down to the length of hill and time taken to reach the top. If you can find a hill that takes 2-3 mins to climb.....and you run it repeatedly at good effort I'm sure you'll more than hit 85%.

Whether it be hills, intervals or a tempo run.....it can taken several minutes to reach 85% of max heartrate or higher.

I don't think you can really test you MHR on a treadmill - you need to find a proper hill outside to get the gradient etc (and remove the little bit of assistance a treadmill gives you).

However, MHR is a slightly different topic to your original post.  For your original post, I suspect you are stagnating as you are repeating the same training.

Try doing a few weeks of base training or similar as you've been concentrating on speed for the last three months, so now you need to add endurance training to boost your perfromance.  Maybe replace your intervals with a tempo session as well?

I'd recommend the "Advanced Marathoning" book by Pfitzinger & Douglas, even if you're not training for a marathon.  It explains the science behind each type of training session and also uses the concept of meso-cycles within a training plan, where instead of looking at 18 weeks of marathon training it breaks it down into four  sub-sections with each section focussing on one particular area.  

85% is around the bottom end of my marathon pace HR once I am into steady running, well inside my aerobic zone. If I was doing intervals to improve my VO2 etc, my heart rate would be well into the low to mid %90s. For hill reps I keep them at about 90seconds long and will be around 90%+ by the end of each rep. I would guess your max heart rate needs to be determined, or more effort is needed.

No problem in doing a max heart rate test on a treadmill - most formal lab tests are carried out on treadmills including VO2, MaxHR, Lactate Threshold. For max heart rate tests on a treadmill, there are lots of protocols available such as the Bruce test. Some protocols need someone else to assist, others you can carry out yourself. It really does help to have someone shout at you (or encourage you?).

Just to update, I did longer less steep hills this weekend (rather than my usual short and steep) and easily got my HR to 85%.  So it was a case of my body getting used to the usual shorter hills.

Perfect! Even under duress (as in the case with short hills), your heartrate can take time to get to the higher levels. Looks like you've found the answer....