Cool, I've seen reports on here of people's preferences for loose exhaust valves.

I wonder if they used a softer valve seat on the 500 heads compared to the 450 head?

Finished the valves this morning and although I've only had a shortish ride, it's made a huge difference. The bike feels much smoother and more responsive. And it starts on the first turn!
peteGS, we actually burnt a couple of valves on the 450 before setting them up a bit loose which seemed to cure it. I stripped and cleaned  the carbs when I rebuilt the bike (had to, somebody at the police workshops had robbed the diaphragm springs) and set the mix ture screwsas per recommendations on the site (2.5 turns IIRC) Other than that the bike is all stock.
gsJack, I did see your valvelogs while searching previously and used your settings as a guide.
Thanks again for all the help guys.

Excellent news! Many years ago when I first got my 450 I had no clue... now that I have a good clue my bike starts and runs like a gem with good valve clearances, properly adjusted plugs and carbs, etc.