Thanks. The recipe above was spot on!
Just made a batch of Kris Dhillon's sauce with 10kg of onions, 500g of garlic, and 500g of ginger, for 25 people.
I made about 8 kg of korma using Ghanna's recipe with the whole spices added, and two portions of the above recipe and was told they were both as good as restaurant meals, so am very pleased.

Making a largish batch of sauce didn't appear to affect the flavour of the curry sauce. I used Kris Dhillon's method of pre-cooking the chicken in the base sauce with turmeric added, but instead of straining out the chicken and throwing away the chicken cooking sauce, I added it to the korma, and I do think it added an extra taste.