Here's another observation of mine, even before I try to make a BIR type chicken biryani from scratch.
The other day I bought a 'mini classics' chilled food tub of chicken tikka makhani from Morrisons. 300g for ?1.29 on the packet and now reduced to 2 for ?2. Highly recommended. It's described as "chicken tikka pieces with a rich creamy tomato sauce on a bed of pilau rice." After 3.5 mins in the microwave you're advised to gently agitate and heat for another 2 mins.
Perhaps I agitated it too much because it ended up as a very moist mix of rice and sauce which struck me as pretty much like a very moist biryani. After all that's how most biryanis are made, i.e. sauce + rice. So for my second delicious 300g tub, I cooked it in the oven instead, as also 'allowed' on the packet. But I added about half a cup of partially pre-cooked basmati rice, to reduce the liquid nature of the final dish and make it even more like a biryani. The rice I added was cooked in a large volume of salted water for just 5 mins, and added until the Morrisons tub was full / level. I didn't stir in the new rice. I just added it on top. I covered it with a pyrex plate to get some sort of seal and put it in the oven for 30 mins at gas Mk4 rather than their suggested 20 mins. I also added a few drops of saffron soaked in warm milk and a couple of small pieces of butter.
The result was a very tasty " chicken tikka biryani" indeed - much better than recent biryanis I've had from any take away.
Th lesson I've learned from this little experiment is that you can't go far wrong with any mix of some curry sauce + rice if you seek to make a biryani.
Do try it.
Regards
George