Audiobooks - buying as MP3 or CD? (specifically Wee Free Men)

I'm looking to get copies of the audiobooks to put onto my daughter's Yoto, meaning I need MP3 copies. I am happy to pay, but I can't seem to find anywhere in the UK offering the audiobooks as MP3 or even CD, only a few eBay entries for the older readings (which seems to have the same copy, but claim to be Tony Robinson or Stephen Briggs). Anyone know if there is a way to get them on CD?

(Does anyone have any comments on which of the versions is better to get?)