Another good evening of work!
With the sprocket finally decided on, I started work on the frame modifications to make it fit, cut out most of the drive side stay, welded in two 1/2x1/2" r.h.s. tubes side by side, braced them to the other side stay in front of the wheel and then bent the remainder of the chain stay in to meet the new part.
That all sounds very simple, but it was not I can assure you! It took about three hours to get right. Getting everything in exactly the right place was a very fine juggling act, I ended up with 2mm (1/12th") clearance between the stay and the tire and 3mm (1/8th") between the stay and the sprocket.
The reason I left slightly more on the sprocket side is that I am not 100% confident I can get the sprocket dead straight on the crank as it is so large and wants to warp from the welding heat, I also think it might pull slightly sideways in low gear due to the extreme angles involved, so left a little more room there in case that does happen.
Here is the first test spin video:
[media=youtube]yFXntrcOiLM[/media]
Really happy with progress, very happy with the bike, even with the monster sprocket it still feels very light and nimble, should be a great bike when it is done! 
Luke.
