Thanks for your helpful reply Mark-sf.
Yes, the plastic inserts in the transport screw mechanisms had broken into lots of pieces and fallen into the bottom of the chassis. I’ve picked them all out as far as I can see, and they don’t seem to be interfering with the suspension. But a complete strip-down is beckoning.
I still can’t figure out why the leaf springs don’t seem strong enough to hold the sub-chassis in suspension. They’re struggling to bring the platter surface level with the surface of the cover plates… Also the platter isn’t quite central in the opening in the main cover; the service manual suggests that the leaf springs can be moved side-to side to adjust this, but on my model no adjustment is possible.
Maybe a strip-down will reveal all! (The label inside my unit says it’s a model 5502.)