Damned Central Heating!

A couple of months ago when it was coldest my central heating stopped igniting. My impression from the sounds it was making was that the thermocouple was bad or the pilot light that would be ignited was dirty or some such so that the flame wasn't close enough to the thermocouple of inform it that there was a real flame there and turn the full flame on.

The repairman came out and said that the pilot and thermocouple was OK and changed out the main gas valve. The heater appeared to work for six weeks and then as the weather got cold again started not lighting again.

The company sent another repairman out and he said that it was the main control board and replaced that.

I am now $1,600 into "repairs" and sure enough - the heater isn't lighting still.

It would appear to me at this time that there is nothing more to replace but the pilot tube and the thermocouple. So I wonder how to convince the repairman that there is something wrong with one of those?