Thanks for the suggestions chaps.
Money is an issue, and I really don't want to put anything permanent down. This is more a corner of fenced off field than tended lawn, and we've just had the wettest "summer" on record, with not much sign of improvement, so things are a mite squidgy.
I know carpet will just go soggy, but it can be stashed in the already wet garage until dryer weather when it could be dried a bit outdoors. Far from ideal I know, but I just know there's something I haven't thought of yet.... like some sort of heavy duty coconut type matting maybe.
I have couple of bins of spare shredded car tyre material which I laid a footpath with last year. Great stuff, but I had to put chicken wire on top when the cat realised what a great place to take a crap each day!
But again, I don't want a permanent track - it'll only be used a couple of times a year hopefully.