Steve,
Have you checked that the chassis and cross head are free from cracks? Plus, check the triangular gusset plates either side near the bulkhead, which are welded to the chassis legs - there should be no rust pushing them away from the chassis...
On later cars these plates are much longer, giving better support. New Elms sorted mine out after years of me trying to trace why St. Morgan's Dance reappeared over a period of some 3 years for no apparent reason.
Cheers,
