I had the similar thing happen a few years ago. It turned out a spider had crawled into the vent & blocked the gas jet in the burner.
I sent the heater back to Trauma (intentional miss-spelling) after demolishing half the van to get the bloody thing out. They fitted a new burner & returned heater & old burner complete with spider, Cost me 75 quid plus postage.
I now make sure the vent cover is in place when the heater is not in use.