I made the Marmalade Pudding Cake from Kitchen, but with some modifications. I used blackberry marmalade instead of orange and used 1 teaspoon honey in addition to the brown sugar. I set my oven (without fan) at 180 C and baked for 25 minutes and checked with a toothpick which came out clean from couple of locations. Therefore I turned the oven off and removed the pan from the oven. However the cake had not set properly in the centre, though it was alright around the edges. Does it mean that I removed it a little too early?
We suspect that the baking time was a little too short. In the recipe (see the link below) the baking time is 30 to 40 minutes so after 25 minutes there is a risk that the centre of the cake was not quite done. When testing the cake with a toothpick/cocoktail stick you should make sure that you check the centre of the cake as this will be the last part to cook through.
You can also carefully press the top of the cake. If the cake is cooked it will have risen but should also feel firm to the touch, if there is some "give" in the centre then again it is probably not quite done and you should bake it for a further 5 to 10 minutes before testing again.
The blackberry marmalade (which we assume is similar to blackberry jam) could also affect the cake as there is soft fruit in the blackberry marmalade which would change the composition of the cake batter slightly and could mean that it takes longer to bake.
However don't waste the cake. You can cut the cooked edges away in long slices and use these for the Orange and Blackberry Trifle on p171 of Kitchen.