From Airline flight crew.
(11/09/2017) about 11am 121.5
We also heard the Mayday call from aircraft over the Wash , he stated he was in a PA 28 Arrow and that the he had rough running engine and was going to try make Marham but didn't know if he could make it . We could hear the rough engine in the background of the transmission.
There was a pause then he said " Oh Christ there's smoke in the cockpit "
We were writing it down to relay and heard that the Military had pick up transmission
I have passed this onto AAIB .