A slow drain rarely clears itself. The water backing up in a basin or shower is usually a build-up of hair, grease and soap that has narrowed the waste line over months.
Start by stopping all water use on that line. Running more water into a blocked pipe only pushes the waste further along and can push it past an access point that a plumber would otherwise use.
Remove the visible trap under the basin if you can reach it, place a bucket underneath and clear what comes out. This solves a surprising number of basin blockages in under ten minutes.
Avoid strong chemical drain cleaners. They damage older pipework, they sit in the trap when the blockage is further down the line, and they make the job unsafe for the person who opens the pipe afterwards.
If the blockage affects more than one fixture, the problem sits in the main waste line and needs rodding from an inspection eye. Call us on 079 612 3046 and describe which fixtures are affected. That single detail tells us what equipment to bring.