I did manage to get the whole mechanism out in the end just before i came to work, i also had the same trouble with the new tumbler chamber not fitting the black outer, this confused me slightly as they are the same size, however i borrowed a micrometer at work & the new tumbler chamber isn't perfectly round, i will try to remedy this with a file when i get home just after lunch.
The new tumbler chamber seems to be produced differently as there are two compression marks each side which i think has squeezed the whole thing out of true, the irregularity seems to be each side of where the brass pins protrude through the casing top & bottom.