Car went into the dealers yesterday, went out for a 40 minute test drive with one of their technicians. Technician couldn't hear anything, but I certainly could. We didn't manage to replicate it at its worst though, and I knew what I was listening for. They were reluctant to do anything with not actually being able to diagnose anything, which is understandable. I checked and confirmed there was a TPI for greasing the wishbone bushes and agreed that I would pay for the TPI to be applied and then claim the cost back if it cured the noise. When I came to collect, they had carried it out under goodwill anyway - bonus!
I headed back to the same stretch of road where earlier that morning it was most evident, and the noise was very much suppressed. If I'm being very picky, it is still there, but if I didn't know what to listen for then there is no way I would pick it up. Hopefully this has cured it, but I've not had chance to get the car out for a longer drive yet.