I'm assuming chdot has posted a pic, but we don't get flickr at the office so I'm somewhat in the dark...
"Unless it has been put in place because a proper Traffic Regulation Order has been passed..."
No, even if a TRO was passed banning cycling, the relevant sign is *NOT* "cyclists dismount". That is an advisory sign by definition and the meaning of the sign (sensibly) doesn't change based on underlying bits of paper having been shuffled around.
If cycling is not permitted there is only one applicable sign and (I understand) that sign MUST be used for the TRO or bylaw to be enforceable. That sign is #951 "cycling prohibited", which I posted earlier and looks like this: