The problem is more pay gap with industry. Even though they tend to pay well by academic standards (e.g. no PhD required, yet pay is much higher than a senior postdoc), the salary is still way below what industry in London or Oxbridge offers.

Furthermore, you tend to be surrounded by non-technical people which may be tough in the long term. Nobody appreciates what you do. Not even your boss, who may know zero about computers.

The bottom line is that positions end up being vacant for long time and tend to be filled by biologists with a bit of coding experience, underqualified IT people or, rarely, really competent individuals that want a break from industry.

I know sufficiently many good programmers who would love to do scientific programming (because they love science) and would immediately accept less pay.

The problem, in my opinion, rather lies in the non-monetary work conditions. For example, in Germany it is nearly impossible to get permanent employment contract when working at a scientific institute and doing something remotely related to scientific work. Even worse: you are not even allowed to work more than 6+6 years (before and after doctorate) in a fixed-term employment position at a scientific institute If this time is over, you are not even allowed to take any non-permanent contract at a scientific institute (the infamous/insane Wissenschaftszeitvertragsgesetz (WissZeitVG)).

No programmer (even if (s)he has a great passion for science) will be willing to work under such extraordinarily bad conditions.

Here it is a bit better. Most positions I know of are de facto permanent. On paper they are not, as most labs go through 5-year funding cycles. So there's a tiny chance of loosing funding. It's quite rare for big labs.

Besides, places like MRC have created permanent research assistant positions. Which are actually permanent and put zero pressure on publications.

As you say, connecting with interested and talented programmers is another problem. I feel that Nature Jobs postings, which were already a big leap forward for rusty uni administrators, are not good enough.

Note that I said should there. How to write maintainable programs seems to be lacking in research area.

And still, you're getting a generic CS undergrad's caliber of work and responsibility which I would say on average is not great. They might not be as familiar with version control, best practices, etc. and could just end up writing code just as bad as the scientists.

I think if hiring a team you would need at least one somewhat experienced full-time software engineer to act as team lead/PM for the other developers, whether fulltime or students.

For sure, talented undergrad CS developers exist, but in my experience there are far fewer of them than many people might think. Experience counts for a lot, especially when trying to deliver even average quality software work.

As for something made by undergrads that is “maintainable” to a standard broadly comparable with an experienced developer? Maybe if you get lucky...

The fact that they need the experience should already tell you that they're not yet competent at what they are needed for. They should be put on low-impact projects that don't matter, or low-impact projects that do matter with solid mentoring. They should not be made to write software for something that is high-impact for you with little to no mentoring.