Try
Amazon.co.uk and look in the new and used.
Flying Training starts at £3.50
Aviation Law & Met. starts at £8.00
Air Nav. starts at £11.50
Aeroplane Tech. starts at £11.00
Human Factors starts at £11.00
Big saving really!
I would recommend buying CAP413 (available free online as a .pdf document) in lieu of Number 7 'RT' and I never bothered with book 5 either because it isn't really pertinent to a PPL unless you want to do and instrument rating after.