Roadside services parking

Is there any legal obligation on the part of service areas to provide parking for caravans?  I know most have some but yesterday we tried to stop at Ferrybridge services but the 5 bays they have were all occupied (one by a transit van with a trailer) and our outfit is too long to park accross normal parking bays.  We had to travel on to Wetherby services where they have more bays but again they were all occupied so we still couldn't stop and our only alternative would have been to leave the A1 and possibly get lost trying to find somewhere to park. 

What do other people do in this situation?