Round the Alster Hof you see loads of runners - seems to be the place to go. Also, http://www.joggingroutes.org/ has a few Hamburg routes (I think the guy behind it is from Hamburg?), though I only tried the Alster Hof one while I was there.
[On a non-running note - incidentally, if you find yourself tired and with your eyes dropping towards the floor... look for these in the pavement http://en.wikipedia.org/wiki/Stolperstein or 'stumbling stones', which are tiny little memorials for the Jews from Hamburg who suffered in WW2. Each little stone represents one person, and is positioned near where they lived or where they worked. Personally, I found these very memorable and very moving (especially when there were large groups clumped together, e.g. outside an old school).]
Just in case anyone stumbles on this who is having the same problem, I tried a massive bombardment of things and it seems to get better.
The number one culprit seemed to be that I was doing much longer runs later at night than I used to, due to changes in my working day and when I could run. So where I could move my runs forward earlier in the day I did.
Also I went to bed with brushed leggings or compression tights on whenever I thought I might be about to have a troublesome sleep, and stretched out my quads before bed, sometimes massaging them too.
Finally I started taking gingko biloba which really seemed to help (placebo effect, anyone?) (I seem to have other circulation problems too, so that's probably the underlying cause of the problem I described in the previous post).
Hope this is helpful for anyone who finds themselves in the same situation.