Hi Bob, from my archives this is what I find:
It seems that the bus mentioned here was a single deck Guy "Conquest" of 1929, It was car 60 OF 3969 and had been a 26 seater with front entrance with a petrol engine. It was converted to forward control, with 32 seats and in 1933, for the British Industries Fair, and ran on compressed town gas. This bus ran until 1935 in that condition and was then converted back to a petrol engined bus but soon withdrawn, as were the other nine of the batch although they remained petrol engined. Bus 60 carried roof boards, similar to those on the Eldon airport buses, proclaiming that it an on town gas.
Between 1932 and 1934 experiments were also done with diesel engines. 442 OJ 5442, a Crossley "Condor" double deck demonstrator was used between 1932 and 1934 (withdrawn 1937), also in 1932 KV 1396, a Daimler COG 5 (Leicester appearance by Brush - not up to Birmingham standards ) was trialled briefly and finally the Guy "Arab" double deck, 208 OC 8208 with Gardner 6LW diesel engine, MCCW bodywork and into service mid 1934.
It seems that the COG5 was the decider that caused the transport committee - obviously guided by MR. A.C. Baker, the general manager - that diesel engines were the corporation buses future. Notable were the decisions to abandon more tram routes (some only had lives extended by WW2) and interest in trolleybuses also waned. Most of the pre-war petrol engined buses had gone not long after the end of the war and the few that survived worked out of Birchfield Road as that became the only place with petrol pumps.
Much of this places the Illustrated London News report a little late, but that may have been a plug for wartime economy and experiment?