The latest on this is that I e-mailed my second supervisor and explained the situation but didn't but all the blame on my main supervisor, just that I understood she was very busy and must not have managed to get round to looking at my work. I asked if she might have some time to have a quick look at my work as I wanted to make sure I had the best version of my chapter to present to the panel.
So far - no reply from her. I did suggest I could e-mail it on Monday to show I did not assume she would be working at the weekend. Watch this space!

Hurrah, my second supervisor has replied and will look at my work. I just hope she gives some positive feedback.

I was in a similar situation recently with my MRes dissertation supervisor.

We hadn't arranged an official date for him to give me feedback on some of my work, although I expected this before he went away on holiday. He ended up going on holiday a week earlier than he told me he was going to, after cancelling an important meeting with me.

I went to see my advisor (who also happens to be the head of my course) and she emailed him for me. Although he was on leave he replied almost straight away, and although I could tell from his reply that he wasn't impressed that I'd gone to my advisor, he sent me my feedback a few days later.

Maybe your second supervisor may be able to get a response and if you're lucky maybe even some feedback from your supervisor. Yes academics are entitled to be away and uncontactable at times, but it really is unprofessional to promise to do something and then not do it.

My second supervisor is actually the more senior and knows me better. She is the chief investigator on the project I am part of so she is quite happy and able to give me feedback.