Delia Smith's creamy homemade mayonnaise should be stored in a screw-top jar in the fridge for no longer than a week.
First of all, put the egg yolks into the basin, add the crushed garlic, mustard powder, salt and a little freshly milled black pepper. Mix all of these together well. Then, holding the groundnut oil in a jug in one hand and an electric hand whisk in the other, adddrop of oil to the egg mixture and whisk this in.
However stupid it may sound, the key to a successful mayonnaise is making sure each drop of oil is thoroughly whisked in before adding the next drop. It won't take all day, because after a few minutes - once you've added several drops of oil - the mixture will begin to thicken and go very stiff and lumpy. When it gets to this stage you need to add a teaspoon of vinegar, which will thin the mixture down.
Now the critical point has passed, you can then begin pouring the oil in a very, very thin but steady stream, keeping the beaters going all the time. When all the oil has been added, taste and add more salt and pepper if it needs it. If you'd like the mayonnaise to be a bit lighter, at this stage addtablespoons of boiling water and whisk it in.
Mayonnaise only curdles when you add the oil too quickly at the beginning. If that happens, don't despair. All you need to do is put a fresh egg yolk into a clean basin, add the curdled mixture to it (drop by drop), then continue adding the rest of the oil as though nothing had happened.