2012 Mondeo Titm X Sport - Cruise Control Driving Me Mad

I've trawled all posts regarding Mondeo CC faults but cannot find a similar problem. Hence new thread. I have an intermittent CC problem. It's been back to the dealer twice and of course when they had it the fault didn't appear. The warranty will run out in May next year and I'm dreading a complete failure after that date. I had the identical CC buttons on my last Mondeo (58 plate) so I do know how to operate it. I press the CC ON button on the left of the wheel and the CC graphic should appear on the dash. This can be done with the engine on and stationary or when on the move. Above 30mph ish the +/- can then be used to set the desired speed. When it works, it works OK, often for many hundreds of miles on a long trip. The problem comes when I press the ON button and nothing happens. On a few occasions the graphic will come on and then immediately go off. I have also noticed that the 'speed limiter' switch which is located below the CC switch (which seldom gets used) also has an intermittent fault whereby the graphic doesn't appear on the dash. During such failures none of the other buttons do anything. It seems that temperature, rain, shine, cold or hot makes no difference. There seems to be no commonality to the fault. There are no error messages on the dash. The dealer tells me it is pointless hooking up the diagnostics unless it is not working at the point of test - of course sods law allows it to work when it's been back to the dealership, twice. Any guidance on the problem or how to deal with the warranty issue on an intermittent fault is gratefully accepted. Thank you.