Well I've certainly prompted some interesting points here, thanks everyone.
I have it on reasonably good authority that it's not illegal to record simply audio, especially as this wasn't for 'sexual gratification'.
I assume she took my drive because she suspects there are recordings on it of her (which there aren't), and the police would only accept such a thing if she made an allegation at the time. I'm going to find out what that allegation was, for surely if it was for audio recordings then no crime has been committed and there are therefore no grounds for her submitting my drive.
It's outrageous that hours of my life, financial details etc. can be handed over like that and there's probably nothing I can do about it.
Who knows, I may be able to get some sense out of her shortly - I'll give it a go, but she's the archetypal woman scorned and is likely to spontaneously combust if I ask her why she's doing all of this.
Citizen's advice was a good suggestion, I'm also going to speak to a lawyer about it. I know I did wrong, but two wrongs don't make a right (to use an old cliche!).
p.s. I presume I'm going to have to shell out on a new drive and a new copy of Windows 7 then...i.e. I can't just use the key again as it's been activated online etc. for one particular PC?