The spark ignition wire, in addition to lighting the burner, serves as a flame proofer to make certain the flame is lit. Over time the wire tends to pull away from the flame front, and the signal to the ingition module gets too weak to keep the gas valve open.
The solution is usually to gently bend the wire back toward the burner flame.
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Thanks John, your right on the mark. I've just discovered the gas pressure is low. That would mean the falme is a bit small to react with the wire.