On 10 May, 00:04, Philip K wrote:
Our quite old, but regularly serviced, Aga has recently sooted up the
complete inside of the lid of the hot (left hand) hotplate. There are
also black soot marks around the ring at the outside of the plate.

The Aga is oil fired.

Any idea what could be causing this? Is it serious, or should a good
service be sufficient to cure it?

Favorite is a build up of carbon on the burner leading to poor burn. A
service will solve this.
Other factors - is the flue clear? Birds might have built a nest in it
I take it double glazing or draughtproofing have not been recently
installed?