Well on Sunday I measured my own 1ph supply at 232.1V using an uncalibrated meter (but I've never had any doubts about its accuracy and always agrees well enough with my other meters), calculating up that should equate to 402V or as you say is over 415V.So what. The actual voltage is usually over 415v. Over 415v is more dangerous than 400v.
Today I've been asked to 'baby sit' a DNO/supplier as an isolator is fitted in advance of fusebox changeover. Currently the voltages are:399,398,401,229,232,230. So just to make winston feel good; I'll agree the UK supply is over 415V
Even though my last couple of installations readings are bang on nominal