You'll need to post a pic, Hollie.
Wow what a monster. Sorry, I can't help but I'm sure someone will be long shortly to tell you what it is.
Have a look here http://www.uksafari.com/caterpillars.htm
Fox moth? That's big - six cms. Is that the same?
The Stycamore moth caterpillar is a very bright orangey brown and one of the few I recognise. I think there are several brown hairy caterpillars and I can't tell one from t'other