Theory is if the voltage exceeds 253 the solar panels close down, and once they close down they are to stay closed down for a set time, also is below minimum voltage they must close down, idea is if they close down for a set time and only reactivate when the voltage is correct, if the supply is lost then the voltage will exceed the limits so if once closed they stay closed for set time, with a street full of solar panels they will not simply feed to each other and maintain a local supply but will fail so as not to cause a danger for some one working on the supply line.
However although the panels paid for by home owners will likely have the correct voltage set, those who hire roof space would be losing money if they close down so if set just one volt high then the home owners own panels will fail first, so it would seem some installers are setting the cut off point a volt too high.
As said in another post the transformers were set to highest voltage which kept them with in the range when under load so think allowed something like 36.8 volts to drop over the length of the run, at 100 amp that gives around 0.368Ω ELI on a TN-C-S supply, but if the transformer is set to 230 volt then the ELI would have to be better than 0.138Ω to be frank I have not seen that many houses with it that low, one solar panel in a line of houses OK, but a whole street of solar panels and there is likely a problem.
However the CFL and LED lamp are if anything more able to stand the voltage variations than the tungsten lamp, and even at 260 volt really there should not be that much of a problem, so although it could be rouge installers with solar panels, I would say more likely something else. My first thought is a poor connection on the supply neutral, this could result in 400 volt peaks, however it would need a voltage logger to see what is going on, it is quite likely that when the meter is connected the voltage is OK, but over the 24 hours you could get peaks.
Again since lights used at night and solar panels do not supply electric at night, it is unlikely to be solar panels.
Large motors can produce over voltage, specially with faulty inverters, I have seen generators over speed and voltage go really high with faults on a tower crane inverter, so if the house is close to an industrial estate that could be causing the problem.
But at end of day looking at a supply fault not some thing inside the house, so it is a phone call to the DNO and let them investigate, also talk to neighbours, are they having the same problem? If the DNO can't help then we start again, but seems likely an DNO fault not something inside the house.
The network wasn't designed for that.