Roadster - where to start?
The simple explanation is that the contra-rotating prop enables more thrust to be produced from the engine than a single prop (think six blades instead of three). To provide the same thrust from a single prop you would need four or five blades (at the time the limit of technology) with a much longer span, thus increasing engine spacing and undercarriage length. There was also no torque effect from each engine, although assymetric laws still applied However, that isn't the whole story as there were also significant weight and engineering penalties as well (of note the Gannet was not strictly speaking a c/r prop on one engine, but two engines with a prop for each!). I'm sure someone else will come onto the thread to explain this in much more detail.
Of more importance was the engines were actually tilted down slightly (I seem to remember the figure of 18 degrees, but that seems a lot) so as to alter the thrust line and hence improve low level performance and fuel usage. There was a lot of quite clever design work to optimise the aircraft for the maritime role, although it still bore much commonality with its Avro predecessors.
However, lets get back to the pictures!