Tonight I have once again been looking at the Manor chassis.  Checking against other stock I found that one reason for the fouling wheels is that I had removed too much material from the chassis and it was now running too low.

the soloution was back to my original plan, fitting the Mitchell frame overlays to the Comet frames.  So far these have only been tacked into place to allow testing, using the break mounting holes for alignment.

however now I had the opposite problem in that the chassis was slightly high and had these unsightly gaps (presumably in part due to the chassis being designed for a Bachmann model with a thick plastic footplate.

At this point I compared with the P4 chassis under my Mitchell 43xx, which imideatly highlighted a problem.  The comet axle holes are about 1mm lower than the Mitchell ones (again presumably to allow OO flanges to clear the thick plastic splashers.  Which leaves 2 options, cut out the hornblocks and resolder in the correct place.  Or remove 1mm from the length of the frames.

the front will also need work, adding a mm or so of meterial to the front frames and raising the clyinders 

This photo shows a Markits wheel in both chassis, the frames are the same height.  The red line shows the top of wheel vs the top of chassis, while the blue line shows the material to be removed / added 

at this point I wish I had spoken to Dave Gene at Wells last year and bought a replacement Mitchell chassis.