BG244 / 240k or whatever its called today is a good place to start as an irregular treatment, that should be used sort of as a midpoint between services to keep the car fresh, otherwise you wont really get more MPG from an additive. You are however probably costing yourself MPG by using a performance fuel in city driving. As such, I would recommend that you are to be no worse off using standard shell or bp fuel, as opposed to their Super Juice. You would save money and MPG will most likely not be affected.
I believe Redex is good as a regular additive (use with every tank). Well priced too. My dad uses it in his land rover and says it sounds a lot smoother.
Certainly good for cleaning out the system but it is only affective when there is actually something to clean up, otherwise all its good for is changing the colour of the fuel in the tank.
Anything that costs more money is often a bad way of trying to save money (if that is the motive)
Increasing the cost of a tankfull of fuel by adding expensive additives or more expensive fuel may just cost you more in the long run
Using a fuel or additive with "detergents" can help clean the pump/ injectors and using it once-in a-while (eg - every few tankfulls) may be more cost effective - be careful - some additives can melt seals if used exessively
Cleaning out the inlet manifold of all the carbon/ soot and fitting an EGR blanking plate or disabling the EGR valve would be a good, low- cost way of increasing the MPG and long term reliability of the car
fitting a decat or DPF delete (if DPF is fitted) would help economy/ long term reliability but it would be a while before you "broke even" and started to save money on the increased MPG - an option is to wait untill your DPF (if fitted) starts "playing up" before ditching it
Again the cost of a remap etc would take a while to recoup in slightly increased MPG for money-saving motives, but if you feel you could use more power it may be a different "kettle of fish"
I run my car on new/ pure veg oil in the summer, it costs about £1 per litre
If you were to add some veg oil (about 10 to 40% - the higher percentage in warmer weather/ when its below 4 degrees C diesel only) you may find your engine is quieter, and its an additive that costs LESS than diesel, instead of a lot more (per litre) (thus saving you money) - pump diesel already contains 5 to 7 % veg oil as a replacement for the sulphor that used to be in diesel (to help lubricate the engine)
There is much talk of adding 2-stroke oil (designed for 2- stroke petrol engines ) to deisel this can help lubricate the injectors/ pump but i would be careful about mixing different types (there are 3 basic types synthetic/ semi-synthetic an castor- based) and how this mixes with diesel & engine oil