IIUI, the ROL claim goes something like this. Under the terms of their original licence from Acorn/E14/Pace, they were required to submit source code changes back to Acorn/E14/Pace for the first few years of the agreement. They did this, and so some of the code that Pace later sold to Castle has ROL-contributed bits in it. Now that the requirement to send source code back has lapsed, ROL are claiming that their licence gives them complete control over their version of RISC OS, and any derivatives of it. It's on this basis that they're claiming control over ROOL's version of RISC OS.
The strange part of this seems to be interpretation of 'derivative'. According to the CVS logs, ROL-contributed sections of the current RISC OS 5 are pretty small. Even if their version of history is right, only a moderate proportion of the RO5 codebase uses their work. Most of it was written by Acorn/E14/Pace/Castle/Tematic/Others independently of ROL.
So even if ROL's claim about which licences are still in force is right (and Peter Wild clearly disputes this too), then I just don't see how ROL's claims add up. RO5 is surely not realistically a 'derivative' of ROL's OS, just a different branch with a small amount of intermingled code. So ROL have no business issuing a licence to, or otherwise interfering with, ROOL.
All of which is not to say that there might not be other issues which cloud the ownership question. Just that the 'derivative' point seems spurious to me.