The message
from David Hearn contains these words:

Similarly, I've had the same thing - most of the fire is okay - but
towards the back in the 2 corners I've got black pots on the concrete or
whatever panels.

What's the best way of removing these marks?

An oxidising flame. That'll get it hot enough to burn off the carbon
/and/ provide sufficient oxygen.

--
Skipweasel
Pay no attention to the man behind the curtain.