Dear Rowan
Apologies for taking so long to reply to your question.  I’ve discovered this morning a number of questions that, for unknown reasons, went into our ‘junk’ folder.
You have probably finished your book by now and my reply is redundant.  However, in the hope that my response is helpful to you or to anybody else in the position you were in when you initially mailed, I would advise that it is always good to have all your material as developed and polished as possible before querying.  That is because, if an agent asks to see the whole thing, you don’t want to be scrabbling round trying to finish artwork or text under time pressure.  Certainly, when it comes to illustration, it would be wise to have two or three finished pieces of artwork, as well as the whole thing illustrated in rough.  (That said, you will need to check each agent’s submission guidelines separately to see what they ask you to send.  Not all agencies ask for the same thing.)  You might also find it useful to bear in mind that, when it comes to a publisher potentially offering for something, they may commission text only because they have an illustrator they would prefer to use.  But that is a few steps ahead at this stage in the process.  
Polly