There should be a spark lead attached to the burner on the left hand side of the grill burner. Shin a tourch in there and you should see it sitting next to the injector were the gas comes in.
To be honest its rare for them to fall off as if it does you will see it hanging loose in there. What they can do is break round the back so instead of sparking at the end its sparking from the break.
In this case you need to remove the oven from the housing but you may need to disconnect the gas first to be able to access this.
Once the oven is out you can remove the back panel and the spark leads will be going to a spark generator unit. You will see all the leads going to there.
You will need to replace the grill spark lead and that should cure the fault.
If you need anymore info just get back to me.