So I have found a little time after stuffing my face with xmas turkey and many other things to do some work!
Took the plunge and cut off the old rear end replaced it with my pre formed new one. Fitted great thank god.

Made up new down tube supports and welded the cross member back in (with holes in) and cleaned it all up, looks quite good if I do say so myself! Here is the result with the tank in place.

Got a new sprocket in the post which was great apart from being the wrong one. Looked nice in gold anodise though so I am looking forward to it's replacement.
Next job is to make the mounts for the seat unit, get on with it as long as the weather stays dry. Which is not very often here!