I've just made a big batch of soup but the curry flavour is too subtle, how can I spice it up at this stage please?
Add more curry powder, or whatever you were using. If it's a powder, it won't need to cook much longer to take up the flavor. If it's a paste or liquid you can just stir it in.
If it's something bigger and you can't grind it, you'd need to cook a while longer to get the flavor out. If you also don't want to cook the soup much longer (if perhaps it has chunks that you don't want to overcook), you could take out a bit of liquid to cook the spices in then add it back in.
It's also possible you've undersalted it, or could use some more richness from stock (or bouillon cubes/paste, but be careful not to oversalt).
You can't simply 'add' more curry powder. Curry powder needs to be cooked in oil to release it's flavor. By simply adding more powder you w ill end up with a rough taste. There are a couple of things you could do depending upon your personal taste preferences. Either fry a small amount of curry powder and then add it to your soup, or fry off a very small amount of chili to add. Alternatively squeeze some lime into your soup, this will help enhance your curry flavor. One last idea, chop some fresh cilantro, stir a small amount into your soup and sprinkle the remainder on before serving.