I have exactly the same problem. I know this sounds ridiculous but the only way I get round this problem is by leaving the grill on full. After a while(sometimes takes up to an hour!!!) the timer re-set display flashes and by pressing the + button, it enables you to ignite the oven. If it flashes again, repeat the process and it SHOULD stay lit. I wrote to rangemaster and they told me the same, that it would cost £100+ labour but they also said that the timer COULD be overridden by dis-engaging the timer . I suggest you contact an elecrician for advice like I am going to.
Good Luck!
Heather Walker, March 2009