Hi Bigeye
It used to be common practice to drop cables down the cavity but now the
practice has been outlawed under current reg's.
That said you will probably find the cables are just hanging in the cavity
if your property is less than 40yrs old as conduit was rarely used after the
60's.
Either way when joining the new cable to the old ensure that there are no
sharp edges; bind the joint with tape to ensure a smooth transition down the
void.
Also get someone to guide the new cable into the cavity and report if it
snags.
Finally don't pull the cable directly into the consumer unit unless it is
remotely isolated by a separate 100a DP switch.
Oh and always pull down (let gravity do the work) and if it snags ;pull it
back up a foot or two and try again.
Good Luck.
Bigyun