Someone mentioned in a previous comment (I think on the last article) that Pace owned the Intellectual Property rights and Castle owned the technology.
Working in academia and with IP's, one thing that I want to clarify is that IP's and technology do not have to be the same thing. IP's are commonly novel ideas or notions, whereas the technology may be just an implementation of the IP.
For example, WIMP maybe the IP of a company, but other companies may have implemented it in their technology (for example Windows, Mac OS, RISC OS, etc). When it comes to licensing, therefore, the issue is whether it is a licence of the IP or a licence of an existing implementation of the IP (technology).
To join in the speculation: when I read the comment that Castle had bought the RISC OS technology, but not the IP, my gut reaction was that this meant they had bought the code off Pace.
RISC OS Ltd may believe, however, that their licence applies to the IP and not this code (especially if this code is a from a different development branch of the OS).