ddenk1: you're wrong. "Exclusive" can mean restricted. If the contract allows ROL to sell RISC OS exclusively for the desktop market, it means the desktop market and nothing else. By way of example, "You have exclusive rights to sell RISC OS on ARM-based desktop computers" means only ROL has the rights to sell RISC OS 4 for the desktop market, whereas "You have the rights to sell RISC OS 4, exclusively for the ARM-based desktop market" means that ROL has been given rights to sell RISC OS 4 *only* for ARM-based desktop machines.
As for your other comment, of course you need solicitors to go over the legal details. However, I meant some things are so obvious that you don't need a solicitor to realise them - but of course you need one to verify and take action.
If RISC OS 5 is derived from 3.XX and not 4, and ROL's contracts gives it exclusive rights to market RISC OS 4 for ARM-based desktop machines, then Castle will not have broken that contract. And there are so many other things that could well be in the contract, you just don't know. For example, maybe it says ROL is the only licencee with exclusive rights, but that the licencer (ie owner) of RISC OS can also sell to that market. Who knows...