If your sink drains are terribly slow at draining water and you live in an older house, the problem may be an accumulation of sludge and grime in the pipes. No matter how many chemicals you send down or plunging you do it will not clear this sludge
The pipes actually need cleaning with pipe cleaners that force the sludge out. This is usally a service we provide but where you can access the pipes yourself it is relatively easy to clear them.
It may require 50 or 100 motions with the pipe cleaner. A licensed plumber will charge you about £150 or, you could attempt it yourself.