Just an update and request for further information.
My set up was :
1. CU -> 2. Cable to -> 3. Cooker switch -> 4. Spur Cable to 5. Oven Outlet Connector -> 6. Cable to -> 7. Oven
I removed item 4. from the Cooker switch, and turned the switch on. I have not had any "tripping of the rcd" in 24 hours. I am therefore beginning to think that it is either the connection of the spur cable to the cooker switch, the connection of the spur cable to the oven outlet connector, or the cable between the two.
I have previously elminated 5,6, and 7 because I had previously disconnected these.
My test is either to have the cooker switch ON or OFF. Previously, with the cooker switch ON, I would get the tripping and with it OFF, I don't get the tripping. By simply removing the spur cable, with the cooker switch ON or OFF I don't get the tripping. Or at least as long as it last another 24 hours, I will be pretty sure this is the case. So it appears that I have the have the cable replaced.
Can I replace the cable myself? It is currently behind the kitchen tiles and behind some plaster (behind the base units). Will I have to keep it behind the plaster or can I surface mount it (it won't be seen as it is behind the units) If I remove the plaster, I hope to be able to "feed" the cable behind existing tiles to the cooker switch.
Also, someone on the boards mentioned that he thought it would be the spur cable because the fault only happened when I put the switch "ON", and with the cooker switch "OFF" no tripping happened. I don't really understand this, because even if the switch is ON or OFF, without an Oven connected how would the RCD know there was a fault? Does current flow and is it detected when the switch is ON even if nothing is connected?