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