How can I get my Potterton Netaheat 10/16 natural gas boiler to light?

My Potterton Netaheat 10/16 natural gas boiler is about 30 years old and has worked well and still is in good condition. I am reluctant to change what has been a very reliable boiler.

1) The pilot light comes on when heat is required however the main gas to the burner does not operate.
2) I notice the high voltage ignition continues to spark about every 7 seconds even though the pilot flame is lighting is this indicating a possible fault or is it normal for the spark to continue?
3) The flue fan is also running.
4) The boiler cover is back on the boiler.
5) The boiler insides are clean and free of any obstruction.

Please help me trace where the problem may be and what component may have become faulty and or be adjusted?
Many thanks.

Phillip
Trim
Co. Meath
Ireland
Tel. 00-353-46-9431515