I'm renovating my bathroom at the moment and currently there is no
supplementary bonding. I have read several threads on uk diy, but I
remain uncertain about some points.

As I understand it

All exposed metal work should be bonded together. This doesn't extend
to things like loo roll holders etc. that are isolated.

Provided the bonding is protected it can be done using 2.5mm cable. In
my case all bonding is going to be carried out within the void under
the bath or within a false wall I am having to build. This will be
accessible, but only by unscrewing an access hatch or bath panel.

I have a light fitted which has a metal case. This is mounted on the
wall, but is fed from a junction box in the false wall. As the junction
box is at the foot of the wall near to where the taps and radiators are
bonded, can I simply connect these to the cpc at the junction box and
avoid having a separate cable from the light down to the radiator.

There is no need to actually link this bonding back to a cpc on the
lighting or shower circuit. But because I have a shower and a light in
the room these will be linked to the bonding anyway.

Please confirm that I have got this right.

Thanks, Tim