Finally put my 18 inch rims onto the car. Tyres that came with them were Cr#p (joys of buying online). Ended up ordering a new set of 255/55 R18 General Grabber A/T shoes. They ended up being a good balance between onroad and off. My 20 inch wheels are in repair mode (damaged a sidewall apparently), so was a good time to get the second set up and running. Hope to have the new wheels on the beach real soon.
Onroad - really doesn't feel any noticeable difference, slightly taller diameter so should correct the -10% speedo differential, I noticed on my 20's. Car is sitting 20mm higher off the black stuff and no signs of any clearance issues. I don't like the look of the higher profile tyres as much (probably largely because the rims aren't black!). More aggressive tread does look quite good though.
They should be on the car for a week, so will try to find some dirt.