Right all sorted now. Without the need to pay a plumber. One pair of old pants in the overflow. Scoop out skanking water. Pour in a pack of soda crystals. Pour in two kettles worth of boiling hot water. Leave to cool. Then scoop out more remainding water and start pumping away with another pair of pants on the plug hole. Flushed it out nicely. Then poured another kettle of boiling water down there just to check it was properly unblocked. Then got the shower on jet function and pointed that down there for a bit...
Must buy a plunger...