Normally you need a spacer for 10 speed cassette on 10 speed hub.
Is it a 9 speed cassette?
Thanks for all your help guys. The guru (Cliff) has spoken and he says no to the spacer as he would be horrified if anyone tried to tighten the cassette to 40NM with a 1mm spacer in place on a Venus 10 speed hub - it would likely strip the thread.
PS What a super bloke, he couldn't have been more helpful.