The problem with these helmets, very much like climbing helmets, is that a dent breaks the integrity of the shell and destroys the ability of the interior of the inner lining (which I know you know already Roy) and the only way to be sure of its ability to maintain protection is to get it X-Rayed. Could be costly though.
Comes down to how much you value your bonce in the end I guess. The only way you'll know for sure is come off and see....and it's too late then for 'should have done's' or 'maybe's'.
About 10 years ago when I first sat my CBT with Two Wheelers in Perth I dropped my brand new helmet on my first day on the concrete floor of the garage used for theory at the HGV testing station at Arran Rd in North Muirton!
I was well choked!