There's a good writeup by Oliver, our head of engineering, about our tech stack on our blog[1] with an accompanying Kubecon talk[2].
TL;DR- Largely Go microservices running on k8s, with http-based RPC calls for synchronous communication, and kafka for asynchronous communication.
As for sending and receiving of this kind of payment message, they are largely async but it does depend on the payment system we're talking about. When we build our own FPS gateway we're going to have to have something to manage "sessions" (TCP connections) which will block waiting for a response to an individual payment messages. Right now our communication with our third party Gateway is via a queue.
[1]: https://monzo.com/blog/2016/09/19/building-a-modern-bank-bac...