That would seem to have sorted it. The steering isn't exactly light but it's certainly lighter roundabouts no longer require so much heaving!
Having cleaned up the old parts I think I found the culprit

That's the upper bearing surface for the thrust bearing. Those lines aren't just lines they're dent's made by the rollers. I'd suggest that when not under load the bearing moved OK but when loaded the rollers picked up on the dent's and the bottom bearing surface then just slid on the locked rollers.
For those interested here are the three types of top mount I found

The bottom row has a lipped lower bearing surface (left) a bearing with a polyamide cage a lipped upper bearing surface and a seal
The middle row is the second type with a flat lower bearing surface a metal caged bearing held captive in the upper bearing surface and a slightly smaller seal
The top one is an oddity - it's a bearing surface with a bonded seal - I couldn't find any other parts to match with it.