It's easy from junction 13. Stay on the A419 through Stonehouse to Stroud, then hook left down the A46 and keep a look out for the Badminton signs, they'll be on your left..
Only use the satnav until you pick up the AA signs as there are one way systems in place for the trials. Once you're on the A46 it is really simple. Bring your wellies, tow rope and snorkel you'll need it!