So I pushed the boom in by 15mm. This resulted in more chain wrap at the RD and more seriously the FD fouling the power side chain tube. So I split the chain and found a small section of 4 links in the chain run with quick links at each end so I removed this section, and then got to work on cutting the chain tube down. Unfortunately whilst I'd tied shoe lace to one end of the chain, I lost the other end and had to spend 30 grubby minutes feeding it back through. So reconnected the chain, put the bike on the stand and ran through the gears only to find there wasn't enough chain for the large-large combination. Bizaarely it was 4 links short so I reinstalled the section I had just removed. Thought I was done but noticed that the lower chain tube was fouling the rear brake and brake noodle when on the granny ring. Which explains the rattle I've been getting from the RD when on the smallest chain ring. There's no obvious soluion to this. Changing to a disk brake or to an IGH would solve it but I don't really want to go there, although I have previously considered a dual-drive. I could try a reversible V-brake so the noodle is on the non-chain side but the cable end might still interfere with the chain tube. I wondered about dropping the chain but that won't work with the suspension fork. Or I could put up with the rattle now I know the cause and that it's only with the granny ring. Will ponder on it.