Yes, coolant sensor shares the same circuit as the EGT. So, if the EGT fails, it can impact the coolant sensor's signal and report failures in both. Interestingly, the EGT is located in a very hostile environment and the tip, which is just a thermocouple, can/will burn away in time. Changing the EGT can be done with a factory special tool, although I haven't done it. The tool is a very long "crowsfoot" type wrench needed to grab the sensor's hex nut which is threaded into the exhaust pipe very similar to an O2 sensor. But without the tool, it is supposed to be a non-starter with engine in.

I'm not one to just replace parts without testing, even with a fault code that points right to it. But from what I understand, the EGT does have a pretty high failure rate on the W12. The "intermittent" failure you're seeing in the codes is consistent with how the EGT is going to fail. I'm guessing it is going "in and out" for you right now, but eventually, when if hard fails, is should put the engine in limp mode.

Hope this is some help.

 

I know this is probably the wrong attitude but at £550 for just the sensors I'm tempted to see if the problem will develop if not then I'll just leave them.

Obviously if I get more CELs I'll get them done