OK, this is the problem, and my solution.

The product type layout table drives the product type settings editing screen.
Without specific entries for the product type that screen has nothing on it.
This makes it a bit useless being able to add a new type.

The new entries have configuration keys, which are effectively the names of the entry lines on that screen, and they have to be unique across the whole table (dammit!). These keys have to match up with something I guess, which means keeping track of the changed names (I've only just thought of that! .

The problem with this is that, when generating a new product type, how do I guarantee unique names? If the type I'm using to copy from has sensibly named keys (ie containing the product type name) it's not too hard, but if not?

Alright, what I'm thinking to do is this :

Generating a new product type I ask for a new name and old type to base the new type on.

I'll find all the layout definitions for the product type that I'm copying from,
if I can find the old product type name in the configuration keys I'll replace it with the new product type name.
Otherwise I'll just add the new product type name!
If someone can point me to a piece of code that finds the common text in a whole load of strings that'd help, but this will work anyway. I think.

Comments welcomed.