I thought I also seen a void on the inside edge of your mold. Sealing voids and making the inside of the mold as smooth as possible, before applying your release agent, will make it just about fall right out of the mold after it hardens.

  I looked in Hugh Piggott's How to Book, earlier today and found that he did add the hardener first.  I guess trying to save a minute during the process is not worth the risk, as you just experienced.

  I'm glad bad weather held me up the last few days, otherwise I'd be singing the same sad tone.  Hope your second attempt is a success!!