Though the fundamental pitch of the tenor trombone is Bb, it is treated as a non-transposing instrument for music writing purposes. So, Les, unless UK practice is different from that in the USA, music written at concert pitch should be fine for the trombonist, as long as it's in the clef he prefers. As a trombone and later a tuba player, when the band was warming up, the "true" Bb instruments (clarinets, cornets, trumpets, etc.) would play a C scale while the trombones, baritones and tubas played a Bb scale. The result would be a scale with all instruments in tune with one another, but at concert pitch of Bb.

What I'm trying to say in all this is that I doubt your trombonist needs any transposition - it's clarinets, trumpets and the like that need it.