Bourton fresh water pump problems

On attempting to prime my water system after being drained for the cold spell the pump runs but will not prime the system, there is a good 8 gallons of water in the tank but nothing comes through the taps, I have tried it with the tap set to hot to refill the water heater and when that failed tried the taps turned to cold but still nothing, where the earlier vans had a pump under the wardrobe this has a submersible one in the water tank which you have no access to without dropping the tank as the access point is at the top which is of course under the floor, it did this once earlier in the year and after setting off home after the break made a stop and just tried again and as sods law dictates it did prime itself.
        Am setting off in the morning so will stop half way and try again  but any ideas would be appreciated, the van is booked in for habitation service in 10 days so if no joy I will get Marquis to check it out, if only A/S had fitted a tank with access it would help, although the tank is fitted with a very small access point at the bottom it is far to small to be of any use, I did hear of one members van that the dealer fitted an access point in the side of a tank which would give access to the pump so I will ask Marquis Devon to do this, will keep you all posted on this.
Don