Well seems I was worrying too soon! I left him in his harness all yesterday evening and he was quite happy to go to sleep in it etc., so this morning I took him for a walk wearing his harness but with the lead clipped to the collar, then after a couple of minutes I just unclipped the lead from the collar and clipped it to the harness instead and after a moments hesitation he was fine and walked like a little star! He even made it past the big scary dustbin truck without freaking!
To answer your question Daiydoo, I had briefly thought about showing, they told me he was pretty good at his ring craft, but to be honest I don't know the first thing about dog showing.