Thank you both very much! I do like the look of the Ruffwear Web Master, because there are lots of reviews from people describing their dogs as 'Houdini' dogs, and also lots from people with dogs with deep chests and small waists. I think that I will get one for Butu, and, if it works, get one for New Puppy. I'll let you know how I get on!
My dogs wear a martingale collar with their harnesses. I snap the leash onto the ring for the martingale and the harness (I use front clip harnesses) - that way if they ever happen to slip the harness, they are still attached with the no-slip martingale collar.