Ah found something:
"September 4, 1970 - Today, after playing to 20,000 fans at the L.A. Forum, Led Zeppelin appear with Fairport Convention at the Troubadour in L.A. to a crowd of only a few hundred. The bands share instruments. Richard Thompson, guitarist for Fairport Convention plays Page's Les Paul, but FC's drummer, Dave Mattacks is hesitant to let Bonham play his drums because of Bonham's reputation as a very powerful drummer. Bonham sits down at the kit and steps on the bass pedal. Mattack watches in horror as his bass drum flies forward a half a foot. After the jam session which lasted almost three hours, the drum heads need a good changing and the toms need a good tuning. After the jam session, Bonham retires to Barney's Beanery, an after-hours bar, where he engages in a drinking contest with Janis Joplin."
Sounds like much fun was had
Wonder if there was any recording of the jam?