Are you on pc and do you uuse addons? If so try disabling all of them, close the game and restart. That has always fixed similar issues for me as well as the "npc missing" bug.
Thanks both - abandoning the quest worked in the end. Interestingly, it made me realise that the dialogue between Valsireeen and the two Saps when you first meet her kicked off again when the quest bugged out, i.e. when I returned to the door after defending the whatsits (technical term), when it corrected itself different dialogue occurred.