If I fill a sponge cake with fresh cream...

[opalescent]

Do I need to add anything to the cream? Vanilla extract? Sugar? Or will Just whipped double cream work as a cake filling (with jam too).

I'd just do whipped double cream with jam as otherwise it would be too sweet for me.

you could, but it's not essential.

A little icing sugar can be nice with a sharper jam choice or just plain fruit

whipped double cream is nice with a good sharp lemon curd to cut through the sweetness.

My mum always said that a spoonful of sugar helped the cream hold its shape but I've grown to suspect it was just because she liked sweet cream...

I have read that a spoonful of icing sugar added to whipped cream helps to stabilize it so that it doesn't ooze out of the cake when you cut it

I usually add some icing sugar (to taste) and a dab of vanilla bean paste (the vanilla seed speckles look so pretty!).