Ginger and Treacle Spiced Traybake

I fancied baking something that I would not normally  try, (this is going to become a more frequent occurrence!) so I decided upon this recipe.
The picture in the book looks lovely, and being the new owner of a tin of black treacle and a jar of stem ginger, I set to work.
Just for your information, should you own a tin of black treacle that is a few years past its sell by date, open with great caution. Someone, who shall remain nameless found me a tin which had not been opened in the back of their cupboard. I wont embarrass them by telling you the date, but lets just say it was as old as Nathan! To say that the lid popped off  would be an understatement! We decided that it would probably not be safe to use, hence the purchase of a new tin!
Anyway, I set to baking this before I took the kids to school, leaving Adam in charge of taking it out at the correct time. When I returned home, the house smelled lovely, but my traybake had a rather a dip in the middle!
It would be rather mean of me to blame Ad, as it is something I have experienced before (see honey and banana loaf), but it was a bit disappointing. I turned it over on the cooling rack, in the hope of disguising the dip, but as it cooled, it dipped back the other way! I'm not sure if it dipped because it did not cook for long enough, or whether it was the treacle in the recipe. Any ideas?
Once it was cooled, I made the icing - icing sugar, and and syrup from the stem ginger. Unfortunately there was not nearly enough icing, so I had to make some more - I would suggest doubling it.
The result was rather delicious, and scored 8.5. The workmen who were here on Friday certainly seemed to enjoy it!
This is definitely a recipe for those that like ginger.