Hi Nigel
I don't know if megaliths get struck by lightning more or less than other pointy things, but there are certainly a couple I know of that have been struck by lightning and split down the middle:
Clava, Inverness-shire
http://www.themodernantiquarian.com/post/22770
Faskally, Perthshire
http://www.themodernantiquarian.com/post/8387
Cheers
Andy