After a bit of help here, My P38 has the 18" wheels and can anyone recomend a suitable tyre for on/off road use, or would it be better to swap them for a set of 16" wheels. I havn't got the room to have two sets of wheels and tyres so I can't go for 1 set for road use and a second set for off road use.

Not much help but have you considered selling the 18" wheels/tyres to help fund a set of 16's ?
You are very limited to 18" all terrain tyres and even then they cost a fortune, whereas 16's are a lot cheaper with better choice


just a thought.................................................