By pure coincidence (I was driving to a meeting) I caught this very interesting discussion on the radio yesterday morning, on Melvyn Bragg's 'In Our Time' on BBC Radio 4.
I can't really stick Melvyn Bragg with his over-fussy and self-referential questioning. But the three subject matter experts and their insights were fascinating and informative.
A really good primer on the ever-mysterious topic of The Picts, if that period interests you...

Replay available on demand (well in the UK anyway, possibly beyond, I'm not sure) on BBC iPlayer...
http://www.bbc.co.uk/programmes/b09cvx7b