Thanks Kern. There are some excellent points there and food for thought. I had considered a pan on a griddle, but I do like the idea of the heat capacity of the water bath. I struggle with this at the moment with my big stock pot on my portable electric hotplate. It makes keeping curds warm while cheddaring difficult. I guess the answer is what I've looked at will work, so it is a matter of buying it and making it work for me.
Will provide an update when I have it. I was looking at the deeper pans, but it seems like most of the bain marie's I've found don't cater to pans that deep. It is a little frustrating. As it is, the 150mm pans are considered 'floaters', but should work with the pan full of milk.