Hi Folks
Thanks to all who posted. Although the reason the lock barrel was falling out was the fact that some of the plastic "fingers", moulded around the hole in the door, had broken I decided to wait and discuss the problem with John of Caravancare Cheshire, when he came to give the 'van its annual service last week.
John suggested and carried out a simple but effective solution of drilling a small hole in the metal "tail" of the lock and putting a split pin through this. This successfuly holds the barrel in place whilst allowing it to turn. An excellent solution which was in fact free as John did not charge me extra. A much better and cheaper solution than a new, unneeded, battery box door at £50.00 plus.
So a big thank you to all who posted and especially to John of Caravancare who solved the problem so effectively and for free!!.
Tim