Thanks for the comments.
An evening in the garage paying with the dirty words in the bike industry (standards and tolerances).
So not all chain ring bolts have the same thread (or diameter) and arm thickness between the outer and middle ring can vary up to 1mm?
Anyway some work with the file to get the carrier to clear the crank and it's on.
Am going to make another chain ring as the inner plates of the chain have a square notch on the outer edges which is catching plus i think some clearance on the tooth profile might be needed.
fitting 28 tooth by Dean-B1, on Flickr
Also thinking you only have to chamfer on side of the teeth as it's only the top third of the cassette that creates a high chain angle.