Rippo,
The Germans converted a number of British vehicles, mostly captured ex-BEF vehicles. Being Germans, they did this thorougly and according to strict guidelines. All the pictures of converted MWs show a bodywork identical to the one in the picture. It is the same body type fitted to other chassis of various manufacturers.
The conclusion I draw is that this is not a WW2 conversion (umbau). It is either a film prop as suggested or possibly an ex-Fire Brigade truck, many of which were converted from army trucks after WW2. Especially Austria got a large number of British and Canadian army trucks to rebuild theit essential services, many of which were in opration decades after the war.
My EUR 0,02 worth. . .
H.