Lots of things can affect fuel consumption and it is possible a combination of factors could be contributing to the drop in the MPG. Tyre pressures and vehicle load can cause increased fuel consumption, so it is worth checking the pressures are correct and removing any unnecessary items from the boot and cabin. Vehicle condition also plays a part - if a service is due then having this carried out may improve things, but it is also possible there is a fault. Damaged or faulty spark plugs, HT leads, coil packs or any one of a number of sensors that monitor and control the air and fuel mixture could result in poor running. It is also worth considering the time of year - in cold temperatures petrol cars take longer to warm up, engine oil tends to be thicker causing more friction and even weather conditions like stronger winds and rain can reduce efficiency. Try the more simple solutions first to see if that helps before having the car inspected for faults.