overflowing drain - help!

Apologies for drain talk, but a suspicious trail of tissue leading away from my external drain cover, coupled with a gurgling toilet leads me to suspect there is something very wrong with my drain.  Any recommendations for drainage sorting out people?

Also - my house is the ground-floor flat  of a house that's been converted into three separate flats.  I'm presuming the external drain serves all three flats, and therefore any work done to it should be paid for by all three properties.  Can anyone advise?