Phil and Roy,
That's a great picture. It seems that Birmingham City Transport had 25 of these AEC Regents converted to run on gas in 1942-43, under instruction from the Ministry of War Transport. The gas was produced by the trailer burning anthracite, to conserve imported fuel. These buses were operated from Perry Barr garage and ran on the 33 service to Kingstanding. Performance was very poor so they were soon withdrawn and converted to normal fuel running.
Brian