FFS you've left it a bit late chap.
I am not a lawyer, but my job requires me to understand some points of road transport law and this would be how I would view it if it involved one of our trucks:
Perhaps the temporary signage was not laid out in accordance with the proper guidelines which lead to some ambiguity and confusion as to where exactly the signs were indicating you to go??
If a road is closed, it is closed to through traffic, but not if you require access to a point beyond that closure, unless of course that point is obstructed by the reason for the closure.
(Remember that proceeding to drive along a closed road may also invalidate your insurance for the length of that closure).
In other words if part of your intended route is beyond the closure and is easily and safely accessible without posing a danger to yourself or other people you should be able to quite legally proceed as intended.
As this happened some time ago, you would have to prove beyond all reasonable doubt what your intentions were at the time and if the signage was ambiguous in its meaning, that Police officer should have been there to help and guide you as to where to go, not throw the book at you right away.
Personally I think you have probably left it too late and may not have enough enough of a valid reason to contest any proceedings against you.