London--Jan. 16--The annual rate of U.K. underlying RPIX inflation fell again in December, mainly because of the effect of cheaper petrol prices, according to figures published by the Office for National Statistics. RPIX, which excludes mortgage interest payments, rose 0.1% on the month and 2.0% on the year, compared with an annual rate of 2.2% in November and forecasts for a rise of around 2.1%. It has now been below the Bank of England's 2.5% target for 21 months in a row.