Steve wrote: [snip]
> But anyway, I've tried to remove it with isopropyl alcohol and whilst
> that's removed most of it, it's still there faintly - as if it's
> soaked into the paintwork.
That's exactly what has happened, I suspect. The solvent in the marker pen will soften paint and allow the wax to penetrate the paint.
Anyone know the best way to remove this - T-Cut?
Probably - it depends how deep the marker ink has got into the paint. I'd make a first attempt using tar remover or white spirit with a few changes of clean paper towel before resorting to T-cut or cutting compound.