If it helps you to know, my 1/2 portion of base sauce fitted into my 7L capacity pressure cooker - this is the pot pictured in the pictures I posted. From them you can see that 3.5Kg of chopped onion bowled over the edge but soon sank down as they softened.
For the record the family are still talking about the curries we had last night.
Regards to all,
Martin
(from this experience I am thinking that I the volume of the pot needs to be double the weight of the onion/carrot/pepper/cabbage used in the recipe - grateful for your wider thoughts)