This where I'm at with solving the rattle. The barrels are bored to +60 by a reputable company that wouldn't do the job until they had a piston to measure; a set of pistons supplied by a reputable company that warned me not to take the barrels anywhere that wanted a piston before they would do the job. "Just tell them to bore it to manufacturer's spec" is what he said but I wasn't brave enough so the boring has been done to suit the GPM pistons. New small end bushes not yet installed but I now have the British-made adjustable reamer and what I think is the essential extra component - the reamer pilot. That should make it possible to ream both bushes 100% in line because the pilot sits in the distant bush holding the reamer straight while the first bush is reamed, and then you swap sides to cut the 2nd bush using the pilot in the first. That's the theory,anyway.
I also found that one cam follower was worn through the hardening, and SRM sent me an exchange Stellited follower by return post.
So far that's £170 for pistons, £70 for rebore, £80 for reamer, £40 for pilot, £20 for bushes, £33 for follower .... £413, which is heading into "hide from my wife" territory, so at the moment I'm doing up the bathroom.....