It seems no-one managed to find the howler on the British Gas advert.
So I'll just have to point it out.
See the three parachutes? The bottom one is titled Boiler and Central Heating, with a spanner. In the middle is Plumbing, with a plunger. At the top is Drains, with a Stilsons wrench.
Now would you fix a drain with a Stilsons wrench? Wouldn't you use a plunger to unblock it? And what would you use a Stilsons pipe wrench for, if not the pipes in your plumbing?
Get it, now?