Many thanks to all who responded to my urgent mascarpone/ricotta query on Saturday.
In the end, after taking a close look at my mascarpone (and checking out recipes on the foodtv website as suggested), all it needed was a couple of drops of vanilla and a spoonful of sugar stirred in, and it made a great accompaniment to the tart and fruity lemon cake and raspberry puree! I think this approach, recommended by several people, turned out better than it would have even had I got the right sort of cheese for the recipe I was trying to follow. I will use this approach again as the result was so tasty and versatile.
But I would also like to take Kate up on her offer of a good tiramisu recipe if it is still open!