I sussed it was definitely the hatch, as the rattle/squeak would stop if I cornered hard enough. So last night, I got the silicone spray out and armed with a cloth, I rubbed down the rubber seal and the surface of the hatch that it touches.
I also sprayed the hinges and also the little rubber blocks that support the hinge arms (which is where I think the noise was actually coming from)
I checked where the glass seal touches the bodywork and there was hardly any marking of the bodywork, so I don't believe that was the culprit.
The result? On this morning's commute, I can now hear all the other little squeaks and rattles that were being drowned out by the (now silent) hatch.....