central locking problem/faulty what??

i have a vauxhall corsa sxi+ on a 56 plate and have had her since new, ive never had a problem till recently when i unlocked my door and pull the handle it relocks itself centrally, the first two times i thought id caught the fob myself but 6am sunday morning driving alone i pulled over to stretch my legs and left the key in the ignition knowing it couldn't lock itself!! it did!! my window was open a fraction but i couldn't reach my key or window button, i was saved by a good Samaritan after 2 hours in the freezing fog, obviously a safety issue i need to get this fixed as i live in an area im not familiar with and im without family or friends so among other things my car is my lifeline, please could someone point me in the right direction of how i can fix it myself if possible.