I had a First Corps Holt tractor in the "might be useful" pile. The same pile also had the two steering mechanisms from the 1/56 JTFM Mark I tanks pictured earlier. They were surplus to requirements. I built the Holt, found a couple of likely lads from the lead pile and cut a couple of bits off the steering gear and presto... a towing limber for a big gun.
Then I remembered this image:
And this from the IWM online photographic collection:
It was what they were pulling that interested me. Big tracked trailers. So I had a go.