For the lack of any better ideas, I've been cleaning up retro edit into a more consistant state and I've now got the control editing the data in a native format. This means theres no real conversion required when lifting the data out an saving it off - or indeed loading data into it.
I still need to go through the application itself as I've changed my mind on several things. For example while its perfectly valid to allow you to edit HiRes and MultiColour more sprites together I'm not going to allow it. This means you dont have to run through a hundred sprites and switch them all to Hires or MCM, and in reality this would hardly ever happen anyway.
I can now draw a sprite okay so what I really need to do is add colour palette selection to let me pick which colour I want to paint with. Once I've done that I can add the features that make editing fun.
I'll need to extend the editing control to allow for character and bitmap mode because I want to use the same control to do all editing. That said, I'm still not sure what I'll be spending my time on over the Christmas break, but for now I'll carry on with this...