Thank you, Terence, for writing this up. Your post prompted me to chase the ICO, and the allocated case officer has just replied:

I have drafted a decision notice that sets out the conclusion of this case. This is currently with my manager for review. Once this decision notice has been finalised, it will be signed and sent to you. It is not possible to estimate when this notice may be issued, but thank you again in anticipation for your patience in the meantime.

I'll keep you posted.