Thanks, I have resubmitted a report on the council website, and this time I have kept a note of the fault report number so I can chase it up if ignored again.
A bit of research on Google suggests there might not be a legal requirement, but it seems insane not to post the height on the bridge itself.
If I get no joy I might contact Network Rail for their opinion.
Their directors get enough in bonuses so they ought to be keeping an eye on such things.