Anyway I have calmed down a bit now ! Got this back from the organisers:-
As the event is multi-terrain it doesn't need to be officially measured
by any of the governing bodies as a road race would have to be and it
doesn't need to be 100% accurate. Any off-road distance is only
approximate as it is impossible to measure uneven ground with the
special calibrations that course-measurers have to use.
Basically I go out around the route with a trundle wheel and measure the
course myself. We do try to make sure that we're as close as possible to
the distance but there will always be discrepancies.
For instance last year we were told by one or two runners with GPS that
we were about 300m over the 10k distance so when we altered the start of
the route this year we thought we would have those 300m to shave off
(presuming that GPS is more accurate way of measuring), but maybe it
appears he was wrong and we have taken too much off.
I hope this helps and sorry if it does come up short, but as I'm sure
you'll appreciate it is very hard to get a truly accurate multi-terrrain
10k.