Try the simple things in order: clear the plughole, flush with hot (not boiling) water and washing-up liquid, plunge properly, then clean out the U-bend. If more than one fixture is backing up at once, stop — that's a main-drain or sewer problem, not a sink problem, and it may not even be yours to pay for. Call 020 4577 2888 to be connected with a local plumber who can tell you.
Don't pour that
Most blockages are self-inflicted, which is good news — they're preventable. In the kitchen, the enemy is fat: cooking oil and grease go down as liquid, cool in the pipe, and harden into a plug that catches everything after it. Scrape fat into a container and bin it. In the bathroom, wet wipes cause more call-outs than almost anything else — including the ones sold as "flushable" — along with cotton wool, sanitary products and nappies. The rule for the toilet is the three Ps: pee, poo and paper, nothing else. This matters everywhere, but it matters double if your home runs on a septic tank, as plenty of rural properties around here do — everything you flush ends up in your own tank, not someone else's problem.
Step 1Clear what you can see
Pull hair, food and debris out of the plughole first — a bent wire or cheap plughole tool gets a surprising amount. Then run hot (not boiling) water with a squirt of washing-up liquid to soften greasy build-up. Boiling water can damage plastic pipework and fittings, so keep it below that.
Step 2Plunge properly
A plunger only works with a seal. Block the overflow hole with a wet cloth, make sure there's enough water in the basin to cover the plunger cup, and pump firmly a dozen times before checking. Most part-blockages give way here.
Step 3Open the U-bend
Put a bucket underneath, unscrew the trap under the sink by hand, and tip out whatever's lodged in it. It's an unglamorous five minutes and it fixes a huge share of kitchen-sink blockages outright. A drain snake or flexible wire can reach a little further into the pipe if the trap itself is clear.
Know whose job the sewer is
If the toilet gurgles when the bath drains, if water backs up in two rooms at once, or if an outside gully or manhole is overflowing, the problem sits in the main drain or a shared sewer — beyond anything a plunger will reach. Here's the part worth knowing before you spend money: in Northern Ireland, public sewers are NI Water's responsibility, while drains inside your boundary that serve only your home are generally yours. Whether you're in Omagh town or out in Drumquin, Trillick or Dromore, where the blockage actually sits decides who deals with it. A plumber can usually locate it quickly and tell you straight whether it's a private job or one to report — which beats paying for work on pipework that was never yours.