What it did not say online is that you need a digital photograph - this is not the normal ID photo - but one that comes with a unique reference. This is now one of the the two options (Digital or ID) that you make when using the picture booth - in my case in Boots the Chemist - Cost £8.00. Returning to the post office with old passport and digital picture, these were handed over - they made a note of the code on the picture, and returned the picture to me. I still had to answer all the usual questions (have you changed your name etc) and then the process was completed.
I am not going anywhere, but my current passport expires in June, so I thought I would get it done now. The process was not quite as straightforward as I thought it would be. Our post office closed some time ago and is now four counters in WHSmith - the queue was longgggggggggggggggggg

PS. I have already received an email saying my passport renewal is being dealt with.