High EGT one cylinder

I have a 912ULS with Airmaster VP prop in a Europa. The engine has 270hrs total from new.

There is a problem with the EGT on cyl 3. At take-off power all 4 EGTs match pretty closely. However at lower power cyl 3 is consistently indicating much hotter than the others. At 5400rpm/26" it's a bit up, at economy cruise 5000rpm/24" it's getting on for 100 deg C higher than the other three which are all around 700C. Cyl 4 tends to be the lowest but not by much. Idling at around 2000rpm no 3 is still just a bit above the others. Cyl 1 doesn't show any odd variations. The engine runs well and smoothly and the plugs (NGK) all looked good when changed last week.

I've switched connections at the indicator to check it isn't an instrument linearisation error. I haven't physically swapped two sensors for fear of damage and I can't see how a thermocouple output could be like this.

Any ideas please?