Fill the sink, a large roasting tin or the bath with water as hot as your hands can tolerate through rubber gloves. Add a generous squeeze of washing-up liquid — the one you use on roasting tins, not the gentle one — and a couple of heaped tablespoons of bicarbonate of soda. Lower the filter in flat so the whole mesh is covered and leave it for fifteen to twenty minutes. You will see the water go cloudy and a faint oily sheen rise to the surface. That is the point.
Then scrub with a washing-up brush or a soft nail brush, working in the direction of the mesh rather than in circles, and paying attention to the corners and the frame where grease pools thickest. Avoid wire wool and anything abrasive: the mesh is fine and will fray, and once it is distorted it never sits flush again. Rinse under hot running water until the water runs clear through the mesh rather than beading on top — that is your test for done.
Dry it fully. Shake it, pat it with a tea towel, then stand it on its edge somewhere warm for half an hour before it goes back. Putting a wet filter back in a hot hood is how you get a slow drip onto the hob.
While the filter is out
This is the only moment you will ever have easy access to the cavity above it, so take two minutes. Wipe the inside housing, the fan guard if you can reach it, and the underside of the hood with a cloth wrung out in hot soapy water, then dry it. Do the same to the front of the hood and the cupboard doors on either side, because they will be carrying the same film. If your kitchen is open-plan, wipe the top edge of the nearest wall units too. Everything that is airborne eventually lands.
