"Budgie Models" by Robert Newson (ISBN 0 948793 02 3) has to be the definitive (and perhaps the only?) work on this toy manufacturer who somehow never quite made it into the really big time, but whose output was prolific enough that their products turn up again and again and again.
The book is split up into three parts: 1) History of the models and the companies which made them, 2) detailed descriptions of the model, and 3) a checklist and index. Part 1 is very good since it explains how Budgie, Morestone, and Modern Products were linked, and part 2 is exceptionally useful since it features a full colour photograph of as-good-as every single model.
The book is long out of print and, being highly-desirable but with a smallish print run, now commands high prices. However, sensibly priced copies do turn up occasionally, so keep your eyes peeled and cast you net wide!
The Amazon page (linking through the secondhand examples) is here: http://www.amazon.co.uk/Budgie-Models-H ... ert+newson.