Retention time on cold days now over 4 hours, though often leave overnight though the wisdom of the latter called into question on finding gallery contents unwilling to move. The explanation soon became apparent when it turned out I had forgotten to give the nozzle of the spigot a good wash with hot water and this had become blocked with yogurt that had set like concrete! The only solution was to strip everything down, separate the gallery from the spigot (revealing an accumulation of semi-hard yogurt in the spigot and the tube close to it), clean the spigot with warm water and remove the blockage. Fermenter re-assembled after checking the gallery and nozzle were once again in working order and a fine harvest once again the following day. Retention time now back to 3-4 hours and no more.