How do I teach a consistent (cockney) accent to my actors?

I am directing Sweeney Todd, and I have to make a decision about accents. We all have the same natural accent (except one, who has a broader-voweled accent) from eastern Ontario.

My pet peeve with accents is that everyone on stage will have a different one, and most of those will change from the beginning of the show to the end. This is what I have observed to be the case with all productions.

If anyone has tips or sites or methods to use that can help me to coach these actors so that they come across like their characters and not like some Monty Python rip-offs, I'd really appreciate it.

Even if the answer is, 'You just have to keep correcting them.', that is helpful.

thanks