The aperture button is closely placed next to the record button on the handle so unintended activation is very likely; one might suggest this arrangement is an ergonomic design fault.
However, lacking any other advice received overnight, I thought I would restore the camera to factory default. But switching on to do this, the problem had gone away. Surprising because this didn't work at the time. Actually, I'm now of the view that there was never a problem, at all.