I was really trying to establish whether the spud had the potential to retain the oil during the cooking process.
Could be. Could be several reasons. Powder particle sizes (of the spice powders), for instance...but I'm sure that's probably pushing it, haha!

I used Delaware or Nadine potatoes (I'm not sure which of the two they were). Both are standard, cheap, run-of-the-mill, readily available, white-skinned spuds. I'd be very surprised if BIRS use anything other than the cheapest spuds available that do the job.
The thing that intrigues me is that I saw the same thing (i.e. oil not separating) with the "cr0 curry base development" which (currently) has no potatoes, capsicum, tomatoes or carrots in it. The oil separated in "curry base 1" but not in "curry base 2" (although it did when AST made it). The only (identified) difference between the two curry bases is half as much salt, in "curry base 2", and a different masala blend. So maybe the answer lies amongst that little lot

Having said that, I'm not too sure why people are hung up about the oil separating though, unless they want to salvage it. It tends to come out in the main dish anyway. It's only interesting in that it has separated for some people, and not others isn't it, thereby indicating a possible difference in procedure from cook-to-cook?
Haven't most people obtained a fairly light orangey-brown colour? I'd think that Bobby's is more likely the exception here (colour wise)? Some paprikas (and chilli powders) do have red food colouring in.
Can you ask Raj why he says the type of spud is important next time you're in there?
And are you returning for other demos (e.g. of accompanying main dishes) sometime?