Well the long and short of it is that etchers have done me a good deal. Given me the scrap etch gratis, as I thought they would and have said they can either send me a new correct sheet, or will do me a deal for the artwork they will need to do on any modifications that need doing at my end, so I have started a mk 0.5 scrap etch test build of the chassis. I can't go too far with it as a lot of the parts are scrap, including annoyingly the full length footplate which is where any doubt currently lies.
However, if things keep fitting together like this, I'll be a very happy chappy... Fingers, toes and any other appendages crossed!
Particularly happy with the drag box and motion bracket.
Doesn't look much like a 4-4-0 at the moment but will come together in time.
JB.