OK, well partial success anyway. Fuzzdog's Mammoth clone bit works very well -- I've added an input level control which means that it doesn't freak out if an active bass is plugged into it. You'll notice the hole where there is supposed to be a Blend control... this is the bit that needs tweaking, at the moment they're not playing nicely together and I need to establish why. Until I've got that sorted, it's still working fine as a plain Mammoth/Mastodon clone.
I'm dead chuffed with the way the artwork came out. I put the pic together in MS Word and then printed it onto an A4 sheet of clear LaserJet label stuff. I'm sure it wouldn't stand up to years of relentless gigging, but it's fine for my purposes.