Replaced the Knock sensors & loom, inlet gaskets, O2 sensors, spark plugs and leads.
The knock sensors that came out looked brand new, but I changed them as I had already purchased them. I did find the cause of the check engine light and the error code for the rear knock sensor voltage. Who ever replaced the knock sensors last had broken the clip and it looks like it has been arcing, the wire going to it was brown. This morning, for the first time since I have owned the car no check engine light!
The car also seems to be running better now.