The plan was really dumb.
Instead of the elaborate scheme they executed, they could simply have loaded a casket with the lead in it and given the guard the 4 keys. He could have switched the gold with the lead. Then they unload the casket at Folkstone and away they go. Having the guard in on it renders all the machinations they indulged rather pointless. Hell, why even use a casket? He could have just put the gold into their luggage.
On top of that, it seems rather obvious that as soon as the gold was found to be missing, the natural assumption would be that it must have been moved to the casket - thus placing the guard under suspicion, and if things happened fast enough, getting the criminals caught.
"I'll book you. I'll book you on something. I'll find something in the book to book you on."