CENTRICa SIGNED the UK's first long-term liquefied natural gas (LNG) contract with Qatar yesterday, to coincide with a trade mission to the Gulf led by Prime Minister David Cameron.  Under the deal, signed in the Emir's palace in the presence of Mr Cameron and Sheikh Hamad bin Khalifa al-Thani, the British energy giant will import 2.4 million tons of LNG into the Isle of Grain terminal in Kent each year for the next three years.  The contract has a total value of ?2bn, using the current gas price, and represents about 10 per cent of the UK gas market, enough to heat 2.5 million homes.  Centrica said the deal is groundbreaking because it is Britain's first ever long-term LNG supply contract. In the past, the company has bought only on a short-term "spot" basis.  It is also vital to efforts to strengthen the UK's energy security as home-grown supplies from the North Sea continental shelf begin to dry up.  "as domestic supplies of gas begin to dwindle, this is an important deal for UK energy security and for our customers," Mark Hanafin, the managing director of Centrica Energy, said. "The important thing is that where we were buying literally month-to-month, now we have secure supplies for three years."  Centrica said that support from the UK Government was an "important factor" in securing the deal, which will be pri