When adjusting lockshields its worth bearing in mind that some have a tiny range over which the adjustment makes any difference.   Sometimes half a turn or less takes it from fully closed to fully open.  This makes getting it 'right' quite tricky. 

If you want to know more about this view Heat Geek on 'valve authority'.

The layout of our house meant that some upstairs radiators had to be virtually off to ensure the flow went on to downstairs or else the bedroom radiator took all the water.

The installer has arranged to come Monday so at least it's pencilled in!

DHW flow temp is completely separate to space heating flow temp.  The offset is added to the target DHW temp to get the FT for DHW.  15K should be ample in principle although the cylinder supplied has a relatively small area coil (1.6 sq m) compared to 3 sq m often recommended.  That said so does the Vaillant 150l cylinder!  Feels to me like something is wrong, suggest you check the pipe temps near the DHW cylinder if you can!

The installer has arranged to come Monday so at least it's pencilled in!

That happened with my installation. Whoever had been sent to the plumbing suppliers to buy various parts got the wrong type of valve. Once the system was running I noticed that one of the upstairs radiators was getting warm when the DHW was being heated. That was convection flow as the circulating pump wasn't running.

As far as I can tell they have been bled (water comes out when I bleed them). Theyre on the same primary as upstairs - 22mm copper running along upstairs, then down. Could anything be done with the 'remaining feed head' setting or is that a non starter?

OK so downstairs is 'after' upstairs.  You may have to throttle down the LSVs upstairs, increase the pump head if thats an option, and open up (if you can) the LSVs downstairs.  As @toodles says there could be an air lock, Vaillant has  a 'purge' function in the installer menu which my installer said was supposed to deal with this (presumably by whacking the water pump up to max).

Its a 150L 'UK Cylinders' heat pump tank. Its got a 3 hour window to heat in from ~36 to 48 deg so shouldn't take that long.

No it shouldn't, nowhere near!

Here is my 200l tank reheating today from/to about the same temperature.  22mm diameter feeds teeing off the 28mm primaries from the heat pump.  I have a 7kW Vaillant.  Something definitely wrong with yours, can you tell what temp the pipes are to and at the DHW tank and what diameter are they?  I suppose its not possible that the installer has fitted a normal 3 way valve instead of a diverter valve (you would know as your heating would be on simultaneously with your DHW) 

Its a 150L 'UK Cylinders' heat pump tank. Its got a 3 hour window to heat in from ~36 to 48 deg so shouldn't take that long.