Investigations suggest that in some ovens – modern ovens, particularly – gas mark 1 is not what it used to be, ie, 275°F (140°C).
Before you start a recipe requiring gas mark 1, please buy an oven thermometer and check the temperature your oven gives on gas mark 1. This is especially important when you are going to the trouble and expense of making Christmas cakes and similar rich fruit cakes! An oven thermometer is an inexpensive piece of equipment stocked by most cookware shops that will ensure you are cooking recipes at the temperature they were tested at. If you find that your gas mark 1 setting is a bit too cool, move the dial halfway to gas mark 2 and test the oven again. If the oven is too hot, adjust the dial a couple of degrees in the other (cooler) direction.
It's also important to remember that the heat within a gas oven will vary from the top (hottest) to the bottom (the coolest part) so place the thermometer on the shelf position Delia says in the recipe you're going to cook, or on the middle shelf if no particular position is specified.