-JdK- Reassemled the 1928 M18 gearbox with new bearings but the rumble was still there ... therefore took the engine completely apart and found the culprit; the timing side main bearing is gone.
New main bearings should not be too difficult to find and I'll have a go at replacing the bigend now that the cranck is on the bench anyway. Further, lets see if the worn camfollowers can be repaired.