I've been contracting in the finance industry in London for seven years (at four firms) and although financial companies are some of the biggest users of open-source software, it's almost unheard of for firms to contribute back to open-source projects. There are a few progressive companies but unfortunately I've not worked at any of them.

Many firms have a strict policy of disallowing any source (open or not) from leaving company premises. Email servers and proxy filters have detectors that will raise an alert if source code is emailed or sent out over HTTP. It's easily a sackable offence and this discourages developers from contributing changes back to open-source: there are simply too many hurdles to jump through in order to do it.

Given your experience, what would you say are the top 5 techs that are being used in this area in London? I was looking at the job market a while back but it's not very transparent. How do you best go about finding a job of this kind?

Sure, here's one example http://www.aplusdev.org/

But the real influence of banking is driving the development of stuff like RDMA over Infiniband, which eventually makes its way into other industries. Everything that "web companies" know (and take for granted) about scalability, performance, resilience, security etc etc comes from either investment banking or pr0n.

There are some examples such as:

https://www.openadaptor.org/

But obviously a lot of it remains behind closed doors, but there certainly some of that sort of thing going on.

Some of it also happens indirectly, for example a bank might design an improvement to the Linux kernel but because they want to remain covered by their support contract with say RedHat they might supply to the patch to RedHat and RedHat will then verify it and merge it into their core release and submit it back upstream, but it would have RedHat's name on it rather than the originating bank who might want to remain anonymous.

Not really what you're thinking of but in similar spirit:

https://ocaml.janestreet.com/

Im not sure about publishing of papers on innovations but there is constant work going on to improve the database systems and few people I know have come up with software programs to check the bankruptcy!