I have now cobbled together a low-pass filter to try to suppress the PA harmonics. This one has a corner frequency of 28MHz (to allow me to use it up to 10m band), using the following components (left to right):

shunt C: 100pF

series L: 12T on T37-6 core, making about 485nH

shunt C: 180pF

series L: 13T on T37-6 core, making about 580nH

shunt C: 180pF

series L: 12T on T37-6 core, making about 485nH

shunt C: 100pF

Performance simulated in LTSpice, and then confirmed with a nanoVNA. I'm now feeling a bit happier about winding the power up without annoying the neighbours...