speirbhean - firstly, many congratulations - I'll keep my fingers crossed they make the right decision.
Secondly, regarding exclusivity my personal take on exclusives is that they suck but if you don't mind giving it, then my suggestion would be to phone up whoever requested your full and ask how long they're looking to hold it for and, if that period is acceptable to you, then restating that the exclusive is for that period in your covering letter. My understanding of exclusives is that when they're asked for, it's generally for 3 months so whilst I'd normally agree with Julie's suggestion, I think it's better to get clarification first.
Regarding the fact that the manuscript's already with a publisher, I think you do need to tell the agent that (if only so s/he knows who it's with when they sign you).
MM