Tizzi - my cat used to get very stressed when travelling, but he wasn't incontinent. Like you, we were moving house - the vet gave us Feliway to try.
We sprayed the inside of the car with it, then took the cat out to sit in the car for a few minutes then brought him back indoors. This happened every day, increasing the length of time he spent in the car, then progressed to going for short drives which gradually got longer. It sounds like a lot of trouble but it worked for him - and we were moving 450 miles away.
The vet wouldn't give a sedative as there would be no-one qualified to supervise him whilst travelling.
Good luck.