I wouldn't worry so much about the pressure switch because if you are getting 5-6 hours of proper use out of it, it can't be that causing the problem. The problem won't be the capacity of your leisure battery, it will be the amount of driving you do to charge it. A simple way to find out if it is a flat battery is when the problem occurs, start the engine on your van and within a couple of minutes the pump should pick up again. If it does, you are simply not doing enough driving to keep your battery charged for a day's work. Solution = charge your battery once or twice a week to keep it topped up.
Another test to see if it is the varistream (if the above does not solve the problem) is to wait until the problem happens again and then simply take the varistream out of circuit (wire the pump directly to the battery) and see if it starts working again. It is not unheard of to get a faulty varistream.