Spymaster? Why not Bard then?
Hmmm... it seems like you've already made your mind up Wippit. But though you dismissed Bard out of hand as being too predictable, I would suggest looking at the class again if it is the sneaking-about type of spy you are interested in. With a good Intelligence, a Bard gets a fair few Skill Points with which to choose a wide range of skills. Unless you expect every infiltration to be a successful sneak, then a good range of social skills is going to be necessary.
Additionally both you and your DM are going to have to tailor things quite a bit for the single PC story. I know from my own experience (I love running one-on-one stories), that you will have a hard time of things, needing to be all things to all men, just to accommodate the demands of adventures. You probably won't max out any skill, for example, but will need a broad range of them so you can stand a chance in many situations. With that in mind, a multi-class, often dismissed as a problem in a group, actually is a boon for a single PC. I run a one-on-one campaign for a single Bard-Wizard PC and this works really well, as he has a chance of answering most situations he encounters.
So I suggest giving the matter some further consideration. But then I commend you on trying a solo game, and wish you fun with the game!