Great! You can find it on archive.org in translation by Ronald Gregor Smith that is not so readable (I am told) as the newer one made by Walter Arnold Kaufmann.He referred me to "I and Thou" (I have read bits of it but must get hold of a copy to read properly).
Dietrich Bonhoeffer makes a great use of "I - Thou" category within a Christian framework in "The Communion of Saints" - again available on archive.org.
It looks like my local library has a copy of the Kaufmann translation. I'll see if I can pick that up. I like a paper book to read on the bus. Bit worried I'd get mugged for an e-reader but no one's going to bother for an old library book!