Just to update this thread. I was only able to get the temp down to 12C as the maximum amount of ice + the mini keg, I could fit into the cool bag was 9.5L. The 8C temp I recorded was probably due to me taking it pretty soon after swapping out the ice bottles and it was the air temp in the bag rather than when everything (the thermal mass of the MK full of beer) had levelled out. So for now I'm going to have to concede defeat.
I say, for now, as this isn't the end of the story. One thing I've recently discovered with my cool brewing bag (basically a MASSIVE cool bag to put a FV + ice bottles in to ferment with),is that when the ambient temp is above a certain temp it takes more ice to cool the temp in the bag down. What this ambient temp is I'm unsure but when the ambient temp is below this critical temp it only takes 1L of ice to drop the temp in the brew bag by 1C. When the amient temp is above this crititical temp it takes 2L of ice to drop the temp in the brew bag by 1C
I'm guessing this might be the same for the picnic cool bag as their just different sizes of exactly the same thing. So I plan on trying this experiment out again in the winter when it only takes 1L of ice to drop the temp 1C in the brew bag.
With the picnic bag I was able to drop the temp by 10 degree C using 9.5L of ice bottles from 22C ambient to 12C in the bag. If what I suspect it true about the critical ambient temp, it means that I should be able to drop the temp by about 20 degrees C with 9.5L of ice. Also the ambient temp in my kitchen will of course be lower. Last winter iirc it was about 18C on the kitchen floor. So if I can drop the temp by 20 degrees in the picnic bag I should be able to lager/cold condition in it. So watch this space and I'll repeat the experiment when my kitchen floor is 20C or lower