NLM13,
You never could. In pre-JAR days, if you had a PPL(A) you could get 5 hours off the PPL(H) course, the theory being that you wouldn't need to do the nav. It was a bit academic as virtually no-one learning in the twitchy, unforgiving (but FUN) R22 ever did the course in the minimum nuber of hours anyway. You were also exempt from all the ground exams except the technical one.
I'm not well up on JAR, but I think you can still theoretically have a few hours reduction on the flying. As for the ground exams, I don't know, but I suspect that if your PPL(A) is an old UK one you'll have to do all of them.
But whatever you have to do, you'll never regret flying helicopters...though your bank manager will. <img src="eek.gif" border="0">