Thanks again for the info in this thread OurFarmersAgent.
Managed to lock the top and middle sections this morning (didn't actually need a slide hammer - just find a suitable screw thread bolt gently tightened with fingers and the bolt dropped with a gentle tug with my fingers so I could manually tighten and engage the locking with a spanner)
Still haven't got the rear section fully locked - manual page talks about needing another 'special tool' on the connection joint of the hydraulic cylinder. Can see roughly where they are in the corners behind the passenger seats but...
Think I've reached the limit of my limited tinkering skills - and given it seems slowly drivable now I'll send it in to get that locked down properly and the root cause of issue investigated.
https://workshop-manuals.com/bmw/3_s...top/page_2252/