Slightly off topic but still LF!

Recently the economy 7 electric meter was changed from a black box to a
grey box with green and red buttons.  The former had a single button
that could be used to select normal and off peak consumption as well as
displaying time. The time was picked up from what I thought was a sub
carrier of Radio 4 long wave or maybe MSF. It was very accurate,
matching time on my GPS. The new unit also displays time but is exactly
10 minutes behind GMT.  Any ideas?

One thought is that the units are set with specific time differentials
so as not to overload the national grid with a single big switch on.

Martin
M0MGM