Thetford service door lock problem

Do you mean getting a new lock barrel and replacing the old one, or refitting the old barrel?  If a new barrel then they are available from caravan dealers,, but also from ebay (https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Thetford+door+lock+barrel&_sacat=0)

My OH says refitting the old barrel is easy - just spray lock barrel with the silicone spray used for Thetford toilet seals and it should slide easily into the door.  However, he says 'Check that the barrel size is exactly the same size as the hole in the new door'.  If it is, and you're still having difficulty, then email Thetford who have a very helpful customer service.  (We've had problems getting a cover to fit a replaced lock as there are two different sizes offered at our caravan dealer and neither fits exactly!)

Sorry I am looking to fit a new barrel to the replaced door.