Indeed, they aren't going to drop the voltage unless they have to. In the main, they have less I^2R losses in their network if they keep the voltagecas high as they can - thus getting to supply more if what they are given to customers instead of heating up their cables !If the supply voltage falls within the limits detailed above the DNO certainly won't be forced to do anything and will do little or nothing unless equipment is being changed within it's normal engineering cycle
There is a bit more to the electricity network than you seem to understand and while micro generation can be a good thing it can also contribute to the local voltage problem you are highlighting if there is a significant amount of localised micro generation which is beyond the DNO's control
But as said, where they get complaints, or monitoring flags it up, they will change tappings where embedded generation (which reduces or can reverse the normal "voltage always reduces as you get further down the network" rule that the networks were originally designed for) causes terminal voltages above the upper limit. They will still aim to stay near the upper bounds though to keep their losses down as stated above.