Roger wrote:
The message
from "Steve Walker" contains these words:

A Petzl Ascender would do the job, eg eBay 120133093026.

I don't think an ascender would suit. They are just the modern
equivalent of the old fashioned prusik loop and, like the prusik loop,
you need to take the load off in order to move up the rope.

As I understood the OP's request, he wanted a one-way pulley - something
which would take and hold the strain. An ascender would fulfill that
perfectly, if deployed correctly, and is widely used to protect both
load-hauling and humans during cave/mountain exploration. The conventional
technique would be to run the rope through a karabiner or pulley, and attach
the ascender to the "pull" side, so that it provides a ratchet effect.

I suspect you'll need to think of this as an 'upside down' location, for
that to make any sense at all. The key point is that you don't have to
relieve the load - it self-adjusts as the free end of the rope is pulled
through it. Quite hard to describe, and I can't see a photo on google
unfortunately.