.....would we be allowed to put in a squirt of tomato purée to make it more tomato-ey? (I find it more sweet than savoury, but don't want to give it up as I like to have variety.)
I would've thought the answer had to be no, sorry Jaycey.
Tomato puree's quite high in carbs. I don't think that realistically it would knock you out of ketosis - but it might make you hungry, and it's not worth taking the risk.
Having said that, you can use pepper, curry powder (up to 1g), herbs to make it taste it bit better.
I found the spicy tomato soup quite sweet too, in fact I'm finding the shakes sweeter than I did at the beginning. I wonder whether it's my body getting used to not having all the sugar I used to eat?