I am getting bad gas mileage

I have a 91 toyta previa that has recently started using way to much fuel, I put £25 in and only got 36 miles in return for it. The car run great, no lack of power, no rattles bangs or any noises what so ever, no smoke, at it last mot there was no issue with em as said in question high fuel usage and it seems to be getting worse, I have checked for a fuel leak and there was none, all fuel pipes in good nick and dry, nothing coming out of the exhaust that shouldnt be.I am baffled