Never fear, my superior intellect solved the problem...
I went to pick the car up this evening, the garage was closed, so I couldn't see anyone except the key guy, but I got in, radio obviously asked for the code so I had a quick shufty in the glove box, just in case and I found a small piece of broken plastic on the glove box tray. I remember when I bought the car it had this bit of plastic in the glove box and I was SO close to binning it...am I glad I didn't!
It contains the code and the serial number for my head unit, and it worked first time!
Cheers for the help, guys!
PS - which CD changers will work with the HU605?