Had my battery pack up. Got a new one. SAFE came up on dash display.

Put in code from handbook. Radio/Cassette now works again. The separate CD unit doesn't and did before. Is it the same code and something as simple as you have to select CD before inputting.    

Its a different code, but should be on your carpass.

Procedure for entering is same, except press 0 (to right of cassette deck) instead of AS

I thought there might be a catch. I lost my carpass in the move to Spain. I kept a note elsewhere of what I thought was the radio code.

I rang the Vauxhall dealer that got most of our business in the UK and astonishingly they gave me free over the phone a four digit radio code and a four digit security code. The "radio" code was the same as the one I had written down.

What does the four digit security code do? Could that be the CD code?

It may be CD code, but more likely its Immbiliser PIN code.  Try calling dealer again, see if you can blag it for free ;)