Sound advice here. I've now taken to watering my paste concentrate down to Martin's suggested ratio. It certainly takes that "sour" edge off the paste. I may try and dilute 1/4 and see what difference that makes. The passata thing is interesting. I've never used the stuff 
Hi CH
As for the Passata, I started using it a while ago instead of blending tomatoes myself for a more traditional version of a butter chicken (I think).
I know it isn't really used in BIR cooking, but I've just stuck with it, and am happy with the results.
I mean it is only the ripe plum toms blended and sieved to get rid of skin and seeds, so not over processed like tom paste/puree.
Plus it's pretty cheap (from lidle again :

) so I usually make a big tub up mixed with the paste, use what I need and freeze the leftover.
Not sure if it makes a difference in the final dish, maybe you could give it a go sometime, and see what you think.
Cheers
Martin
PS I think 1 to 4 with water may be a little to thin/weak IMHO.