Thanks for all the feedback and clarification, everyone.

I'm wondering if Bruce has quartered the amounts of ingredients he saw being used in the kitchen. It seems reasonable, as he lists 1/4 each of lemon and bunched coriander, and 8 onions x 4 = 32, which is in line with the amount that MarkJ was told by his friendly chef. So if Bruce saw a whole tin of tomatoes being used, he might have got home and opened a tin to find 4 or so.
So I'll definitely try just one in the next batch.
Out of interest, here's a summary of the alterations I've made to the BE sauce which resulted in my best effort so far.
One pint of oil instead of 250 mls, for reclaiming - but no ghee
3 & 3/4 pints of water instead of two (and more added at the end, to get required consistency)
Two black cardamoms (removed before pureeing) and 1tsp methi leaves added with the spices in stage 2 - but only 1/2 tsp of ajwain seeds
Two carrots instead of four
One slightly undersized green pepper instead of half each of red and green pepper
One whole tin of tomatoes (whoops - but it worked)
Half a 2oz pack of cut coriander - as many stalks as possible rather than leaves (1/4 of a bunch the size sold by my local Tesco would be equivalent to a whole 2oz pack, at least)
No yoghurt
and I didn't add the Tarka.
Left overnight before pureeing, oil replaced after and the whole base brought back up to a boil then simmered until the oil resurfaced for reclaiming. Cooled immediately in a sink of cold water.
I think the ajwain does need to be there, but (as everyone agrees) nowhere near the amount Bruce specifies. The smell I've identified as being 'sort of toffee-ish' in bought curries is, I think, very close to the aroma produced by the combination of cooked green pepper, black cardamom, methi
and ajwain seeds. But they've never been mentioned by any of the chefs you guys have talked to (though thyme has, which does have a similar aroma).
I'm having issues with the celery - I think it's responsible for a gritty texture in the finished base. It seems to remain quite fibrous, no matter how long I puree for, and even when I've boiled everything for half an hour longer than the recipe suggests.
If anyone's wondering how I arrived at 3 & 3/4 pints of water, I weighed all the prepared ingredients and then compared that to the combined weights of the ingredients listed in KD's recipe. KD uses 2 & 1/2 pints of water, but produces much less sauce. So I scaled the water up pro rata so the proportion to BE's ingredients was the same (if that makes sense).
This produced nearly five litres of sauce - enough for 10 of my curries - once the extra water had also been added at the end.