Dave P wrote:

My question is what to do with the switched live wire. If I simply
disconnect it then there's a live wire floating around. I've tried to trace
the wire back to a junction box to remove it with no avail.

It has been suggested to me to wire it along side the permanent live into
the fan, which is ringing alarm bells somewhere.....

That would make the light come on all the time. Is a 3-pole fan
isolator switch fitted anywhere? If so you could disconnect the
outgoing sw. L feed there. Otherwise it's best to disconnect it at the
light end if you can - there'll either be a junction box in the loft or
a terminal strip in the luminaire. You should still insulate the end
though - fold the cut wire end back on itself an apply a few turns of
red (or brown) tape, or use heatshrink sleeving.

--
Andy