The cost per litre for this shampoo as delivered is actually the same as the Auto Express best buy Ultimate Snow Foam shampoo - ie £21 for 5 litres from available sources including delivery (it is currently unavailable at Amazon). The Alto Kew Combi was said to be very good by Auto Express, but was marked down because it was supposed to be more expensive.
The problem is that according to the Combi instructions you should use this shampoo neat in the spray bottle. I have a C120.5 with spray bottle supplied, and if I did this it would use around 300ml per wash. The Ultimate is supposed to be diluted around 1 to 5 with water before putting into the bottle. That would make a huge difference on cost per wash.
I tried the Combi at 1 to 5 but this was probably too dilute for my sprayer. I now use it neat but move the spray around the car more quickly. I also spray under the wheel arches and underbody to remove salt and mud, so I probably use more than most people anyway. When this bottle is finished I will try using Halfords shampoo diluted and report back.