Cheers for the info.
I know about the abrasive properties of t-cut etc.. (I recently used toothpaste to remove a scratch from sister in laws pug )

I did have to keep changing cloths too. There was that much crud it would form into sticky globs, like rubbing old sellotape gunk off something.

Clay sounds like a good investment but I had worried about the whole 'pick up' problem, asuming objects would stick to (or in) a clay bar easier than a cloth. I might just get one of those megs clay kits with the spray.