Thanks all so far, tempted by the Screwfix item, sulphuric acid good idea, we rejuvinated the car batteries when I worked in the lab, but best to point out the dangers to those not used to it. Can't easily dismantle as the pipework runs under the floor. I have gone past the plunger vacuum method as dad appears to have been washing food off plates down the sink for a couple of years now without me twigging what he was doing, my excuse I didn't want to crowd him & he only announced the problem when the sink stopped emptying - bless him.