[Arran]

 I think that I may own one of these, either that or mine is a Marconi Model 159. In any event the set is from about 1940 or 41, and is definitely Canadian built. One thing to be aware of is that there are some rubber/gutta percha covered leads coming out of the power transformer, which is why I haven't tried powering up my set yet. These sets are a bit unique in that they have a three gang tuning condenser but no RF amplifier stage, and they use a 6K8 triode-hexode converter tube rather then a pentagrid convertor like a 6A8/6SA7 like most sets from then would. As to why it motorboats it could be a number of things, like if you put a 6Q7G in place of a 6Q7 without adding a shield, or an open ground on a bypass capacitor, or a bypass capacitor missing or connected incorrectly.
 You already noted the bias cell, make sure that the polarity is correct, modern watch batteries have the case as the positive so you would need to swap the connections around to use one in place of the Mallory carbon zinc cell. A trick to get rid of the cell is to place a resistor between the cathode of the tube/s it's connected to and ground to make it more positive then the grid, just copy the value from a set that uses that sort of setup, and it will need a bypass cap in parallel with it of about .047 mf.
Regards
Arran