Have you tasted it? No good just guessing.
Having said that, I’ve just found a Mary Berry recipe online and if it’s the one you’re using, it’s a lot of sugar in the apples and not much in the crumble, so it might balance out.
I’d probably try adding another cooking apple if I wanted the apple part less sweet, but you could add lemon juice and zest, a little at a time, tasting as you go along, until it tastes right.
I don’t like her crumble topping recipe either, btw. I think - assuming I’m on the right recipe - that’s a little too much butter to the amount of flour.
Keeping up with grandchildren who have more energy than sense
