back ache

Stretch those hamstrings mate... Stretch them morning noon and night even on your non cycling days. It may take a while but by god it helps. Do a google search for hamstring stretching exercises and go from there. I had the same problem for ages thinking that I could just ride it out over the week but I got no improvement til I got my hammy's sorted. Hope it helps.

Droppy

stretch the hamstrings, but also stretch the quads and the HIP FLEXORS. They are my major source of back discomfort.

Perfect timing for a good example. About a couple of months back for maybe 2-3 weeks I did not stretch and did tons of intervals in a very aero position. Since then my back was getting sore on and off. 3 weeks back all of a sudden it got worse to a point that I could not ride. A week back I was walking around in a store partially limping when it got so worse that I just couldnt move.
I have been on bed rest since and it has taken me a full week to start moving. Now I can ride maybe after 3-4 days. The chiropractor was surprised at how stiff my hamstrings had gotten. There were some muscles in my upper back that felt like a rock every morning from the compensation.

Moral of the story: Stretch and stretch some more!

Yes stretching the hamstrings has helped me with my back pain too. While you're at it stretch the whole body and do some abdominal exercises.

From my experience, your saddle is too high. Even if you are "fitted" your seat may be too high. 1/2 an inch or less difference in your saddle height can fix sore backs.