Hi Neil- Happy new year's eve-

On another note- - - (food for thought) perhaps there's a tinge of moisture underneath the cap and rotor...acting in unison with the scenario: a) that they haven't been changed too recently or b) if they are due to be changed based upon observed corrosion condition- - - you'll get symptoms similar to yours

The caps and rotors on the E500 was just changed for me based upon this scenario described. I had a bit of a rough idle (no check engine light, no limp mode)

hope this helps
-fad