What could Make my Punto drop fuel consumption?

hi there,
i have a mk2 53 plate fiat punto and as a general rule i used to get about 450 miles to £40 - £45 with prices fluctuating so much. over the last couple of weeks ive noticed im only getting about 250 miles which is a massive drop and is starting to cost me alot of money.
theres no major performance problems or noises or warning lights going on. and nothing majorly wrong as far as i know as was serviced about a month and a half ago and MOT'd 5 months ago. can anybody please give me a rough idea of what it could be all help is greatly appreciated

sjudge
April 2010