The way that the ignition system of a gas hob normally works is that when you turn on the tap to any of the burners, you have to push the knob in for the spark generator to work. (The sparks will occur at every burner -not just the one you've turned on). You have to keep the control knob pressed in for 10-15 seconds until a flame detection thermocouple at the burner head heats up sufficiently to hold open the gas supply solenoid to that burner. This is a safety feature to ensure that gas will only pass to the burner when the gas alight.
When you let go of the control knob, and the flame remains alight, the stream of ignition sparks should cease.
If simply switching on the electric power to the hob causes a continuous stream of sparks, it seems to me that something on the extra high tension output circuit from the spark generator is completing the circuit and telling the generator to emit sparks continuously. It could simply be an ignition lead that's come adrift from one of the burner control taps and is shorting out, or it could be that one of the momentary switches which you depress when you press a control knob in has a short circuit (ie, is stuck in the 'on' position).
Much depends on what level of competence you have. The fault should be capable of being diagnosed with the power turned off, using an Ohm Meter to test the continuity of the output circuit from the spark generator, but do do that you do need a modicum of electronics knowledge and how to use a multi-meter to diagnose and rectify faults.
Any competent domestic appliance technician would, I feel, be able to quickly diagnose and rectify the problem.
Though only a minute current flows in the spark circuit, it's at several thousand volts, (rather like a car ignition circuit), so you ride at your own risk!
Hope that might help a bit Steve.
