A have some pannier racks like these (not my photo):
http://forum.ctc.org.uk/viewtopic.php?p=123600&sid=b4ad612e0925093fbadda99bc2185bb8#123600
which came from Halfords, and a rack bag that clips onto them.
I'd like to get another rack bag that fits on in the same way,
but Halfords seem to have stopped selling them.

(The bag has a plastic baseplate that clips into the L-shaped holes
on the top plate of the pannier, and a sprung hook at the back that
latches .)

The thread with the photos links to 
http://www.cl-massload.com.tw/products.php?pid=78
but if anyone happens to know of a UK stockist, I suspect that
would be a lot easier.

(Westcountry Recumbents price list does include "GS/Massload pannier rack"
as an option. Carol?)