Just popped back, a mere TWO YEARS after finishing this project, with one more model! This was always planned to be part of the project but I ran out of steam and didn't do it. I just found it in the back of a cupboard and being "between armies" I thought I'd repair, assemble and paint it as I'd intended.
Many of the repairs and modifications I made are now invisible - there is a plastic subframe underneath helping to support the broken and lead-rotted structure, along with a lot of metal pins. I also made a few tweaks to how the giant skull fits to make room for the crossbow orcs, and added little wooden platforms so they could stand in those locations.
The biggest change is the crew - I used these old Citadel orcs to replace the original Ral Partha Orcs, which are a completely different style.
Maybe I could add one more Orc stood at the back of the machine to operate the trebuchet - which actually works, by the way - but for now I'll call this finished, AGAIN!