If i work out your consumption of 160 miles as 3/4 of a tank and prorate it up to a full tank doing 213 miles, , it does take it slightly less than what i was getting.
But not significantly different to make me think there is a car problem. To maximise efficiency in a KA, you need to use as high a gear as you can. Even at 30mph, my KA prefered being driven in 5th. Also, driving fast on Motorway does effect the amount of miles. My motorway sweet spot did seem to be arond 70-72. The car was happy to go faster than that, but that seemed to be the point where the effificency dropped.
KA's are good little cars, but they arent that fuel efficient.
If you are really looking for savings, you'd be better getting a fiesta diesel. Thats what i swapped to, and i now get about 500 per tank!! plus Car tax of �35 pa, and congestion charge exempt!!!