Hoverflies

Hoverfly larvae always appear on my pepper plants in the greenhouse. I found them eating mealy cabbage aphid on red cabbage last year too. It's interesting to watch them feed, they grab an aphid then sort of rear up on their back legs and thrash around while they suck the juice out.

I always plant alyssum and gem marigolds in or around the greenhouse, since they are easy to grow, manageable in size and attract a broad range of predatory insects like ladybirds and parasitic wasps. They definitely like coriander flowers (there's also a small bee which really enjoys them), and fennel is good too.

Here's a more extensive list of plants - https://tomclothier.hort.net/page44.html