Pete, thanks for the input.
a friend of mine on the same outing had the roclites and coped better than my trailrocs so I am suprised that you consider them not good in the mud, however its good to have another opinion! Nice also to hear that the mudclaws are better on rock than i expected given the info on the invo8 technology page about hard surfaces.
I am attempting one day of the Jurassic Coast Challange (yes i know, only one day, what a wimp!) at the end of march and know from experience that the terrain is a mixture of very slippy mud, grass, chalk paths, steep hills, rock, sandy paths and also some tarmac as we go through swanage seafront! Trying to find one shoe fits all! 
Hmm, Looks like a death match between X-talon and Mudclaws then 