As others have noted, it needed extra cooking time. I didn't have limes, so I used lemons. And I didn't have fresh coriander, so I used ground coriander.
The finished product was nice, but a bit bland for our taste. So I spiced it up slightly with some Jalapeno pepper sauce, and extra sea salt and pepper. It was then delicious, both DH and I enjoyed it, served with steamed rice, a dollop of yogurt and plum chutney.
It was runnier than I'm used to with Dahl, more in the direction of soup. So I cooked the leftovers (lots) with more lentils to thicken it up, then put it into the freezer for future meals. Hopefully Lentil Dahl freezes well!