Steve,
I did what you suggest & got the Thom books either second hand or via the internet book sellers who have a lot of new stock at knock down prices, one came from Browns in Hull I recall.
Lady Whirls II is right about getting a new Air Law book, I did my training on Microlights so had to get the Microlight version anyway, but you need to be right up to date there.
BTW - Personal preference was for the RT book in the Airlife series as I found CAP413 hard going (I went straight to the end of the story, turned out it was the ATCO did it!). At the RT exam I was asked if I'd read any of CAP413 to which I replied that I hadn't. When the examiner saw the book he commented that it was the content of CAP413, just written in terms us microlighters could understand.