Things quickly got very messy, so I decided to fix what I'd done so far and gave it a good old spray. BUT...
...it turns out that permanent marker pen is not so permanent after all, and it especially doesn't like fixative, which acts as a solvent. The black bled copiously into the picture before my eyes.
If it was just the image, I might just be able to do something to make a feature of the effect, maybe, but the signature (important for the auction) is beyond redemption.
So, now I'd better buy a new canvas to replace the foul and useless mess I've created, and start afresh.