3.9 lpg disco with mallory ignition problems

[smokeyone]

I am enquiring for a mate of mine. He recently bought a used disco and all was fine until two weeks ago. First the engine turned over but would not start but go back two hours later and it fires. Then the problem occurs more frequently and then misfires a great deal but eventually settles down.
Now it will not start at all. Electricity is reaching the ballast resister and the wiring seems like this
http://www.v8engines.com/faq-mall.htm#Wiring
No spark at all ...

Any suggestions on where to start to find the problem please.
Thanks very much...