Thanks for the replies - I'll get back in the engine this evening and add a little more pre-load - hopefully that'll fix the issue. If not, I'll try tightening the chain - a little more - to see if that fixes it.
I have to say though that after putting over 300 hours on the bike in a year, I have some doubts about the chain slap - but I've been wrong plenty of times before (hopefully I'll be wrong again on this one because I've got a race coming up this weekend).
Thanks again!
-Al