I did the same thing wrong with Cadbury. What worked for me was having a lure in the hand I was guiding him with, but rewarding out of the other free hand. So he never actually ate the 'lure' treat. Eventually I stopped having a lure in my hand, but kept the hand closed so he thought I had something, and continued to reward from the free hand. Didn't take long for him to cotton on and I was able to open the hand.
Btw, I have real trouble getting Merlin to perform a down in new places, because apparently it is a position that makes dogs feel vulnerable. So, for instance, this can be a problem when we have to do the table at an agility comp, though so far none of the organisations I have attended are using the table. But just something I have to bear in mind.