For what its worth.. my 75 is a 2001MY with 49,000 on the clock
so hardly super high mielage...
If you want to save money on the 1.8 ZS.. I would start looking at smaller cars (lighter is better for economy)... like, say, a Shitty... errr City Rover... or an MG F - or that sort of size...
My MG F (1.8i) is one of, if not the, cheapest cars to run I have ever had
- it does 400 - 500 miles to a tank when there is only the driver in... changes when you have a passeneger though!
Mind you the 125cc Bike I had used to do 350 miles to £10 of fuel... Maybe its worth looking at a bike alongside your ZS?