Just an update really, thanks to all for suggestions, most pointing to EGR valve (Multi is 1.9 JTD). Spoke to St Dogmaels dealership by phone and they thought it would be EGR too, but said it would need diagnostic check and then book in 2nd day for whatever work. However, Cross Hands dealership is closer to home, so it went there today for diagnostic check. On the way, car decided to develop fault of revving by itself!
Service manager there scathing of Fiat Forum. Insisted that EGR valve fault would be picked up on diagnostic check. I duly went away and returned 1 hour later, to be told that :-
1. Rear wiper needed replacing £27.40
2. Alternator belt was cracked and there was a bearing noise from either cambelt bearings or alternator belt bearings. Cost would be £170.24 for alternator belt and tensioners, and £287.09 for cambelt and idlers.
3. Discs were worn and corroded on inside, needed changing £289.74
4. Throttle pedal potentiometer and brake pedal switch were showing up on diagnostic check and needed replacing £284.47.
Service manager advised that it was their opinion therefore that the brake pedal switch was the cause of the loss of power as it could cause the brakes to be applied when they should not be.
I asked why the car was suffering symptoms such as the smoking when the power did come back (and catapult the car forward!) He advised that it probably needed servicing. I told him that it had recently been serviced, he said that the service should have included the addition of an engine additive to clear the engine and if this had not been done, this was probably the problem. He was insistent that car was unroadworthy with potentiometer fault and that this should be attended to immediately, and generally seemed to try to put the fear of God into me!
My comments on the above are:
1. Rear wiper seems fine to me, no smears etc!
2. Accept if alternator belt is cracked it needs replacing, and as I have no evidence of cambelt change accept that over 5 years has passed (although only c. 50k), so needs changing, but why are labour costs so high when both being done at same time?
3. Husband had changed pads recently and discs fine then, also fine at MOT!
4. Accept throttle pedal potentiometer is cause of revving, but could anyone advise if brake pedal switch is likely to be linked (there are no symptoms of car braking without my applying pressure to brake), and from what printout of diagnostic check seems to say, these are either/or causes of "P1555 - Accelerator brake feasibility, Signal not valid"
None of the above seem to explain symptoms of no power when engine is cold for about 100 yards, followed by burst of smoke from exhaust and car then flying forward! Diagnosic check readout says "EGR status - Not active" am I right to assume that this means it has not been tested / cannot be tested?