Hi JanW,
According to my copy of 'A Pictorial Record of Southern Locomotives' (ISBN 0 86093 443 8 ), the C class has a locomotive wheelbase of 8' by 8'6, and a driving wheel diameter of 5'2. The tender has a wheelbase of 6'6 by 6'6 and a wheel diameter of 4'.
This makes the wheelbase of the C class the same as the new (and old Poole versions of) Farish 4F and Jinty locomotives. Not having a set of drawings for the Deans Goods to hand (I do have some somewhere though!), I suspect the wheelbase would be around 7'3 by 8'3 - I know that the GWR seemed to favour that wheelbase for several of its 0-6-0 wheel arrangements and didn't they use something like 7'6 by 7'6 for their tenders?
I seem to remember an article in the NGJ a few issues back where somebody used the N Brass etches along with a new Jinty to make a Deans Goods. I think that predated Union Mills's release.
Personally, I'd have a go at detailing up the Union Mills version simply because it will had the correct wheelbase; their newer tender drives also seem to be much more controllable than the old ones. Well my D16/3 is, I've got no need for the Deans Goods so can't really comment on that one but I'd be surprised if it wasn't as good. I do agree with you about the wheels though but there are ways to make them look better, just see some of Dr Als work!
Anyway, hope the wheelbase information helps.