Doing more homework on this subject (forgive my lack of experience - this project is supposed to be my initiation in this field!) it suggests that if I do in fact mismatch the baud then errors on the bus should be expected.
This is derived from what I read here:https://community.nxp.com/thread/447898
"If the node with the 250k baud rate is NOT in listen-only mode, it will detect an error within each message and will send Error Flag, destroying the bus traffic. Because the other nodes (500k) detect the error caused by the Error Flag, they discard the current message and increase their error counters."
So my observation at the minute is that in all likelihood my device is not being configured properly because this does describe the issues being found. Yet I do send requests from my PC that should set it to listen mode, so maybe there is a mistake with how I am initialising or setting the mode.
So might be a code issue after all. it's food for thought.