Unless the cambelt was in immediate or imminent danger of snapping and you had indicated this fact, (which neither you or the garage could reasonably ascertain without a strip down & full inspection). Legally it would not be deemed unreasonable for the car to have been driven the relatively small 30 mile distance as it was designed, to the Renault dealer for the routine work to be carried out, especially as you accepted this fact when leaving it and you were aware the car would need to be driven there.
Bear in mind this cambelt could just as easily have broken whilst you were personally taking it to the Ford garage, it was just sheer pot luck that it didn't. These belts simply do not have any definitive mileage, age or usage conditions before they snap, therefore neither the Ford garage or yourself could reasonably have forecast that it would have broken within 30 or even the next 3000 miles?
To have any legal redress you would first need to be able to prove that the damage has been somehow caused by their negligence whilst it was in their care, or their incompetence has contributed to the damage. If you had voiced your concern upon leaving the car over the short longevity left in the cambelt (if you had known?) or left instruction that you would rather the car was transported instead on a recovery vehicle, then you would have had a very strong legal case.
Having said the above, that would only be applicable if the cambelt did actually break en-route and they are telling you the full truth?
If in fact you believe the car did actually make it to Renault and there has been an attempt to cover up shoddy workmanship due to the Renault dealer's incorrect procedure or fitting of any parts, then as huckster points out, the AA / RAC or another reputable garage should be able to identify and report whether or not the belt did actually break whilst the car was under normal usage, or if they consider inept fitting at the Renault dealer was the true cause of the problem.
If you can prove, or if other experts consider this type of damage could only have been caused by Renault then you have a virtual rock solid legal case !!
It could also be possible that the Ford garage have attempted to do this work on your Renault for which they were not adequately trained and are trying to make out it ocurred on route to the Renault dealer. I would check with the alleged Renault dealer to see if your car was indeed booked in. If it does transpire that there was no appointment or their records do not support that version, then you can question the Ford garage as to why the cambelt snapped whilst it was on it's way to a non-existant appointment ???