Provided the bonding is protected it can be done using 2.5mm cable.

Use 4mm. Providing protection is normally not practicable.

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.

An adjacent junction box is an acceptable location for running the
supplementary bonding.

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.

Personally, I would regard the junction box as too far from the light, so
unacceptable unless the CPC was 4mm, which is VERY unlikely. However, it
would be acceptable if the CPC was protected, such as being in a metallic
conduit, and otherwise met supplementary bonding regulations for minimum
size.

There is no need to actually link this bonding back to a cpc on the
lighting or shower circuit.

There is if you have lighting or a shower.

Christian.