Before you spend out on an ignitor its definitely worth checking a few other things first.
One of the recommended maintenance tasks is to periodically remove the heaters from cover(when the heater is turned off and is cold) and vacuum to remove dust etc.
With the front cover off, it's worth just turning the gas control on far enough to cause the ignitor to spark. Look carefully along the run of the two wires between the gas valve and the ignitor. One of the wires should be trapped under a small clamp to pierce the wires insulation and to ground it to the chassis of the heater.
If you see any evidence of the spark jumping to either of these wires, it means the ignitors ground connection has been compromised. Re tighten the wire clamp until the spark can be heard but not seen on either of these wires.
Try relighting the heater to see if it has cured the problem.
If it still continues to spark, then we need to check if the flame detection circuit is actually working. The ignitor uses the conductive property of a gas flame to show when a flame is present. The flame should bridge the gap between the spark electrode and the metal burner bar, and it will ground a high impedance voltage which signals the presence of a good flame.
To do this you will need a length of insulated electrical wire with crocodile clips on each end .
Make sure the gas control is in the OFF position. and clip one crocodile clip to some bare metal on the heater chassis.
Carefully pull the High Tension spark cable off the ignitor bloc spade connection. Keep your hands clear of the spade terminal unless you want a 30kV jump. Don't worry its very unlikely to harm you if you do get zapped but its not advisable if you have any heart problems.
Tune the gas control on enough to start the ignitor. When the ignitor fires you should see the spark jumping from the spade. Carefully take the second crocodile clip and move it towards the HT spade terminal - Don't worry as long as the the other crocodile clip is grounded you shouldn't receive shock. At some point about 5mm from the HT spark should jump to the crocodile clip.
Move the crocodile clip so it touches the HT spade, at which point the ignitor should stop firing. if it continues to operate when the crocodile clip is touching then that confirms the ignitor is faulty. If it stops it proves the ignitors flame detection system is working correctly, and obviously there must be some other fault with the electrode or the burner
As both eth electrode and the burner are inside the working parts of the heater you should not attempt to investigate unless you are qualified to do so.
Some times blowing a compressed air line up through the heaters under floor air intake, might blow out any cobwebs, or other debris. This might correct the problem.
But if not then as its now most likely an internal problem you should arrange to have the heater serviced by an approved technician.