How do I change the curve of an envelope?

Flabberbob territory he he..

I can't remember off the top of my head but this is a modulation trick whee you get the envelope to modulate itself or something?

Yep, it's called recursive modulation. Try modulating envelope's attack or decay with the envelope itself (e.g. Filter Env -> Filter Env Attack).
A couple of caveats:
- It's not perfect; while it does affect the envelope curve, it won't give you a real exponential curve, for instance.
- the displayed envelope shape (as in your screenshot) is not affected.