Wilson is perhaps the best remembered today - and the book seems to be readily available on ebay / book websites but at prices from £50+.
The other 3 are rarer and harder to obtain - good luck hunting them all down!
The truth about Wilson by WSK Webb, Red Lion Library (from DC Thomson)
Killer Slade of the pony express by Matt Carson, Red Lion Library (from DC Thomson)
The Goalmaker by Jimmy Baker, Red Lion Library (from DC Thomson)
Braddock and the flying tigers by George Bourne, Red Lion Library (from DC Thomson)