I know the issue with condensing using the clef of the first staff in the condensing group, but I have both tenor and bass written in the bass clef and the condensed part is correctly in the bass clef. My problem is, the bass part is an octave high in the condensed part (but in the correct octave when not condensed). What’s happening? 
That’s interesting. Normally I would have expected the Tenor part to be too low in the combo rather than the bass too high.
The tenor instrument will be the one used for the condensed staff, so provided you’ve used an actual tenor instrument, then the octave transposition is coming from there. You’d have to use two bass voice instruments to avoid this problem at the moment. Obviously we are planning further changes in this area.
Ok, that seems to work - I’ve assigned a bass voice but labelled it “Tenor” and assigned it to a tenor playback sound, and it all ends up in the right octave when condensed. Can anything be done about the tessitura? I’m getting lots of warnings because Dorico thinks I’m being really cruel to some of the bass singers! (I know I could turn them off, but it would be useful to have them if they were correct).
Oh - and can I set a different clef for part layouts? Although I’m confused whether the octave transposition is associated with the clef or the voice…
The octave transposition is part of the Tenor sound, not the notation. Notate a middle C (in any clef) and the HALion Tenor will sound an octave lower.