bluegeranium
I don’t use baking powder only self raising flour which was Homepride (I always use McDougalls but it was much cheaper. I will test the oven temperature but the oven isn’t very old so I doubt it’s that. I’ve made hundreds of cakes and never had this problem.
I've got a new oven and haven't tested it for cakes yet but it has a fan facility as well as conventional (I think, have to investigate a bit more yet).
I'm sure years ago my cakes were better baked in a conventional oven than in the fan oven I had until recently.
Did you use soft margarine or block, bluegeranium?
I usually use butter but have bought Stork soft margarine to try next time.