Re: It strikes me...
It varies from bank to bank.
After the Millennium, Barclays (my main experience) spent significant amounts of money and a lot of effort consolidating as many systems as they could onto a maintainable platform (they called it project "Stretch"). The aim was to eliminate as many of the old systems as they could.
This spanned the Mainframe, and most of the UNIX platforms they ran.
The main aim was to get all the systems running on a smaller number of larger, more cost-efficient systems that were virtualised/shared and could be bought in volume, and managed more easily (sound familiar - they were well ahead of the curve, and doing this before Hyper-V and VMWare were invented or regularly deployed). But a side effect was that the systems could be maintained more easily, and could be subject to regular OS updates,
They did not completely succeed. Sometimes, the specifics of code that cold not run on more modern versions of the respective OS (I remember Sybase and Informix causing some problems). Sometimes they required specific hardware that no longer worked on modern systems (hardware encryption cards were the most serious problem IIRC). But where this happened, the applications were flagged as in serious need of replacement, and the internal businesses warned that they were in significant danger of losing their service and breaching audit requirements.
But the penetration was more than 80% when I left that project in 2005, and got better after that time, I'm led to believe.
In general, the only stuff that really got left behind was the international banking systems for minor countries (like African countries), where upgrading the remote systems was complex and difficult because of their location, so the local systems were kept largely in line and on the same hardware. But even this was being addressed. In all of these cases, there was an explicit risk evaluation done by the internal auditors, so the respective internal business managers were well aware of the risks they were taking.
So legacy systems were the exception rather than the rule, and were known about. I'm sure that most of the other large banks in the UK are the same. Certainly not what is being implied by some commentators.