As I mentioned, its a guesture
They will hav to claim off their own insurance anyway and should your own insurance dem that the accident wasnt your fault, then their insurance will have to accept the entire claim from them
If it was deemed to be negligent on your part by your insurers, then their insurers could get their costs back from yours and your neighbours would not be hit with any excess ( im pretty sure )
Either way, if you werent negligent , but their flat as damaged and they then had to go through the tit for tat insurance rigmarole, it would be nice to make sure they werent out of pocket for their excess, as whatever happened, it certainly ant their fault that they got flooded.
Sueing a plumber ? very difficult to be honest - difficult to foresnsically identify the root cause of the problem and the plumbers influence on it. for example of you get a spanking new boiler fitted an a few weeks later, the rads blow - which they nearly always will cos of the pressures etc of the new boiler vs. the old decrepit rusted rads - this aint the plumbers fault, as the boiler itself didnt blow.