Thank you all for the comments. I really enjoyed making this one and wasn't completely sure I could pull it off until it was base coated!
Is the new 'driver' a cut'n'shunt using another miniature, or wholly greenstuff*?
The driver uses the original mini as a basis with the following mods:
I cut the long beard right back and cleaned up the chest of the mini adding a GS waistcoat, scarf and belt buckle.
As the original pilot was a dwarf, his arms, legs and torso were too short. I cut the arms off and replaced the upper portion (above his gloves) with upper arms from a 1:48 scale tamiya mini. I lowered his "crotch" with GS to increase the length of his body. I cut his feet and foot-rests off and made some GS knee pads . There were pistons running down the front of the walker's legs which I carved out and replaced with GS lower legs and a new pair of feet.
His top hat is from an Ironclad Miniatures figure.
The walker itself has been altered by removing the arms and re-positioning them using plastic sprue. Some plastic rod was used to make the pistons.
The Gatling is from a Games Workshop Empire plastic mini.
Now I've done one it should be reasonably straightforward to do another (without a beard this time!).