> Bearing in mind this is likely decades old C code that can't be rewritten or replaced because the entirety of the UK's aviation runs on it.

It's new code, from 2018 :) Quote from the report:

> An FPRSA sub-system has existed in NATS for many years and in 2018 the previous FPRSA sub- system was replaced with new hardware and software manufactured by Frequentis AG, one of the leading global ATC System providers.