Great work Phil and thanks Admin.
SS I haven't found it tricky as I haven't tried (yet), but my view from reading all the posts is that several others did. Onions cut too small or blended beforehand, too much water, not hot enough, instantaneous transition to burnt, bitterness, wrong brand of Tandoori Masala, etc etc. There also seemed to be a confusion between frying and caramelisation.
JerryM really experimented and at one point considered this to be the goods, but then toned it down to being an optional inclusion. Still, he rated it which is what makes me think it's worth a try at least. I will definitely not be able to obtain TRS Masala.
As I've said, this is similar to Mm's paste, which I've made a couple of times now quite easily, but it is also a long cooking time process