In article o.uk,
says...
The frequency is very important, it's maintained at 50Hz +/- 1% ie
49.5 to 50.5 Hz nut National Grid work to an operational limit of +/-
0.4% (+/- 0.2Hz).


There's also a close tolerance on the total number of cycles in the day,
too.

Had a quick fossick for it, but found this instead...

http://www.nationalgrid.com/uk/Elect...ncy/Freq60.htm

--
Skipweasel - never knowingly understood.