Does your oven have a timer which can be set to start and stop oven cooking? If so, the problem could also be with the timer because it regulates the oven's gas flow and ignition.
I have the same problem with my gas oven, a Rangemaster 110. I have been told by the manufacturer that if the timer has broken, it can be overriden by disconnecting the timer altogether thus enabling the gas flow and igntion to work independantly. You could try asking an electrician.
Hope this helps.
Heather Walker, March 2009