how can i fix starting of my mg tf ?

i have a 2005 mg tf it has stood over xmas battry dead, fitted new battery turnes over fine starts for about 3 seconds then stops( imoboliser cuts in i think) the key fob will not work, replaced fob battery but no diferent no alarm light flashes its as if the imobaliser not clearing. it has been started over the xmas period both tines had the same problem had to start it about 4 to 5 times cutting out after 3 secs but eventualy running ok but now wont can any one help me?