Paul,
Another thought . . . I know that Clive Gande was producing some really terrific quality vehicles a while back. I only have one, but it is quite exceptional (see below). Maybe you should ask him if he made an artillery truck or tractor.
Louis
Yes, Clive's range is called Grey Goose Collectables, and was produced mainly by Dave Love of Mountford fame, with figures partly by Alan Caton, not long before he passed away.
Some of the production side was handled by RP World Models in Nottingham.
They are a clever combination of resin, lead and brass etch/fret, to give you a solid feeling model with less weight and cost.
A CJB style, solid lead version would have been an absolute brick to post.
http://www.grey-goose.co.uk/product-categories/category/Western-Front/1116/1118/
They made a whole series of Leyland lorries, with different bodies, but no a gun tractor, as they weren't used in that role - Though there is an 13-pounder AA one, similar to more familiar Thornycroft lorry depicted by Britains Premier/Charles Biggs
(Which incidentally, you can still get as a kit, without the Charles Biggs crew, from RP World Models)
The original figures were sculpted by Andrew Stadden, who works for many makers including Tradition, and the now defunct Empire Miniatures and Sarum Soldiers.
Leyland lorries have a local interest for me, as they bought back all the lorries from the British Army at the end of WWI, and reconditioned them in the former Hawker aircraft factory in Ham, about 1/2 mile from where I live.
The idea was that, by preventing badly maintained wartime trucks hitting the civilian market in poor repair, they'd be able to limit damage done to their brand, and control prices by regulating supply.
Clive issued a few in a WWI desert finish - Not sure what he has left, or if any further production runs can be made, as Dave is retired, and Alan is no longer with us, and I think some of the moulds were sold off with his estate by accident.
http://www.grey-goose.co.uk/product-categories/category/Desert-Campaign/1116/1119/