Mixed singing (ie instruments and voices) sessions are hard to find, and the pubs tend to turf customers out at closing time (even for the afternoon!). Try to find out what the Barden of England is doing, he is usually at the best sessions. He started a nice impromptu one in the Tartan Frigate (I think I have the pub name slightly wrong) last year (after the electric guest was over). Also keep an eye out for where Hartley Morris are gathering, when the electric bands are over they may well sing. The Sunday morning session in the Nelson is for religious unaccompanied song only, and the shanty session will frown if an instrument appears. It's also worth fathoming where Skinners Rats and/or Slattery are as they also tend to be bellwethers of song sessions.