OK, so I was blind to the obvious …
SIP uses the physical number of the header pin rather than the OperSprinkler convention of using the logical GPIO number. See attached diagram for the mapping.
This also explain the constant high/switching on physical pins 11, 13 and 15 as these are used by OS to control the onboard shift register hardware.
So before you try any of the above tests, just re-connect the relay board to a set of pins marked with GREEN arrows and use the corresponding GPIO number in the Station settings.