Neil Williams ) gurgled happily, sounding
much like they were saying :

Aren't there speed cameras in the overhead sign gantries on the M25?

Yes, but it is reputed (I have never tested) that they trigger only
when the variable limit is set to something other than 70mph.

AIUI, they are set even when the limit is 70, but at a reasonably high
(into three figures) threshold.