Encouraged by the comments here I decided to give this a go, and to follow the instructions to the letter, despite reservations over putting the raw lentils plus water straight into a salty sauce. I used the basic red lentils, (Natco) that would normally be turning to a mushy dhal after about 20 minutes simmering in plain water.
At the end of the recommended simmering time, 30 - 40 minutes they were still hard, as in gritty. After an hour and a half they had just begun to soften, but by that time we were starving so dived in. The sauce was indeed delicious, although lacking the smooth texture expected of a dhansak and as feared there were digestive "issues" throughout the night.
In fact the sauce was so nice I'm going to have another go but next time adding the lentils pre-cooked.
Rob