Sounds nasty Keiran, not the normal road rash. Enjoy the rest .
I'd replace the helmet if it took a whack. The impact resistance is damaged in a crash so wouldn't work as well next time (hopefully there isn't one). If it only took a light knock it should be okay. Proabably if the outer thin plastic doesn't crack it'll be okay, if it is cracked then the knock was hard enough to damage the inner.