Well, I fished the last bit out of the kitchen bin as I was very curious to see what happened.
Microwaved it for 40 seconds, nothing happened. Did a further 10 seconds, bit of a change. Did 20 seconds more and wow, it puffed up and completely changed!
Let it cool down and gave it to dog, who after sniffing and poking it suspiciously for 20 seconds, crunched his was through this wonderful honeycombed thing, not a crumb of it left.
Brill! (Sorry bednobs, no help with your microwave problem)