The only thing that I've found in 40c that looks decent in mud are Vittoria terreno wet G+. Anything else is too gravel/mixed terrain oriented with too small knobs too tightly packed.
I recommend these, they don’t look very aggressive when compared to tyres like the Limus from Challenge, but they have great levels of grip, clear mud well and roll nicely on firmer surfaces.
The only thing that I've found in 40c that looks decent in mud are Vittoria terreno wet G+. Anything else is too gravel/mixed terrain oriented with too small knobs too tightly packed.