wrote:
Since I exfoliated the more padded area of my anatomy on a stretch of
wet tarmac,

Sounds painful.

my rear brake has been acting up. I have a full and
unrestricted range of moment in the lever (Shimano 105) but seem
unable to apply sufficient pressure to the brake.

The brake mechanism (and pads) are fine. However, the shifter took a
knock when I fell and scraped a bit along the road.

You have either stretched that cable, or it has slipped through the nut
at the actual brake.

There is a screw on the lever where the cable exits. you need to turn
this so it move out of the lever, and this will tighten the cable.

If you need to keep adjusting this, or do not get enough adjustment from
this, you need to pull a bit of cable through the nut at the brake.