I've now done what I ought to have done in the beginning and made a sketch !
There are a number of small shelves (not shown) fixed on the face of the flue wall, so IMHO that corner is a bit of a 'dead air' area. Obviously it can't be fixed to the front of the wardrobes, as they are all sliding doors.
I'm thinking of putting it about half-way along the longer interior wall as marked. It may be a bit close to the door ??, but there are various other things on the wall between that position and the flue, so it's the best clear space. Otherwise it would have to be immediately beside the bed on one side or the other, in which case it would probably deafen me if it went off!