my sime combi boiler will not stay alite?

my sime (combi friendly boiler) will not stay alite once i have lit it. to lite it i have to push a circular nob in and wait a few seconds for the spark to ingage
. once alite i have to hold for a second and then slowly turn the nob clockwise and hold for a moment and then release but then the pilot light goes out. can any body help me with this problem many thanks