your car seems to have bad tuned kit....simple tuning will improve things InshaAllah....beside may i ask:
1)whats your engine oil change interval, which engine oil do you use.
2) when was the last time you changed your air-filter?? and whats the interval of its change??
3) when did you check/changed your plugs??
4) maintain recommended tyre pressure.
for tuning in islamabad you should go to Enercon outlet... (i have never been there but) i have heard its good for CNG kits tunning etc.(location: Abpara/melody??).. Compared to Islamabad, Rawalpindi is full of good CNG tune up points. try Kandaan CNG pump near New Town Police station in commercial market rawalpindi... or try National CNG station at National Market (chandni chowk).
a well tuned/well maintained Santro should easily give double the mileage you mentioned.