The quantity of bread for the My Mother's Bread Sauce recipe seems too much, 800 grams is nearly a full loaf and there’s no way to get it in a pan let alone make a sauce. We had to discard half the bread and add loads more milk and water. The result was a huge volume of very thick bread paste that tasted okay. Surely it’s 80 grams (3 or 4 slices) not 0.8 kg?
Nigella's recipe for My Mother's Bread Sauce (from NIGELLA CHRISTMAS) is, as the title suggests, a recipe based on Nigella's mother's own recipe. Everyone has a personal preference for the thickness of bread sauce and this version is a thicker, spoonable sauce with a fairly large volume (as can be seen from the photo).
It is possible to halve or quarter the recipe for smaller qunatities. If you prefer a thinner, more pourable sauce then you could reduce the amount of bread to 200-250g, though this will not be the version served by Nigella's Mother. However 80g of breadcrumbs is too little to thicken 1 litre of milk and the sauce would be very thin.