Took the battery out, the acid level was a bit low so I topped it up but it still would not charge. I tried a different charger, still no joy. I phoned for a new battery and was quoted £100. I gave charging one last try, I got out my really old charger and low and behold it started charging. After a couple of hours charging, I reconnected one of the new chargers and it worked.
I can only assume that these modern chargers have some devices that stop them charging completely flat batteries (probably to save then from overheating or something). Today I put the battery back in the van and it works, the van started first time. I am glad I hung on to my old charger.