citroen relay wont start?

Got a 2002 relay 1800lwb cold weather we're having started the van up struggled a bit with the battery but did start, drove it for 30mins and noticed the immoboliser light on the dashboard was on, stopped outside the factory jumped back in an hour later van will turn over but battery drains after 10secs put a booster pack on but still wont start but turns over fast and the immoboliser light stays on any suggestions?