Thanks again Melanie.
Well it seems I've learned something so glaringly obvious I really do feel all gumby and shamed. A clock signal of 5hz will take significantly longer to tick over than one of 4mhz (like somewhere in the vicinity of 3999995 times slower). Long enough to make someone think something isn't clocking!! I can't believe I didn't pick that one up. (Maybe an excuse is coding at 3:00 AM ) Thanks for your help and I'll really try to think things through before asking such silly qu's in the future.

Cheers