Two pages at the Classic Lightweights site:
The story of Hill Special Cycle Frames And a 1951 Road / Path model in the readers’ bikes section
The website of the National Cycle Collection at Llandrindod Wells
Velo-Retro – based in Pasadena, but with many links to other great vintage lightweight bikes
The excellent Classic Rendezvous Site, includes information on many of the other classic marques with a dedicated Hill Special page here
A website dedicated to the fascinating and mysterious world of Bike Jumbles