I just had a quick look on the Argos website. Both of mine are Prestige which is the best brand I reckon. You measure the cooking time in "whistles". So if I was cooking chana daal, I would soak daal for an hour, put daal plus water into the pressure cooker (about 2 times volume of water to daal) and then turn on cooker to medium to high heat. Once the steam has released (whistled) 3 or 4 times (15 mins). I would take off the heat and leave the pressure cooker alone for a bit to allow the pressure to level. To check you can open the pressure cooker, slightly lift the whistle at the top, if no steam comes out, take off the lid. If you want to cook the daal more, put lid back on and cook for a further 1 or 2 whistles.
If you over cook, your daal can become mushy and lose the shape but it's still edible (and definitely easier to digest).