Hi Paul,
Sadly, there's nothing really available with 'a bit of maths' - it's either layman all the way, or it's textbook. I guess there's Penrose's Road to Reality, but the maths in that 'layman' book is worse than most textbooks
Good layman books are by Brian Greene - Elegant Universe, The Fabric of the Cosmos, etc. Also Hawking's two: Brief History and Universe in a Nutshell - worth chewing through but certainly not as understandable as Greene.
Something a bit more basic and real Cosmology-based, rather than the theoretical stuff is 'Bang! The Complete History of the Universe' by no less than Brian May of Queen fame
A great great book none-the-less.