Update: Well its been an interesting day with the 500's issues, I called the auto electrical garage - they were baffled, had replaced the crank sensor but didnt fix the issue, so they refitted the previous one, and advised they were at a loss - seemed to be a timing issue between crank and cam sensor? and gave me the number of an fiat specialist in Bury and said to try them.
Got the car back in the workshop and managed to test out the crank sensor, so resistance around 1180, and with a little (lot) of help from my brother managed to test the AC voltage of the sensor - so a constant reading of around 5.5v at idle, and rose to about 20v at ~2,000 rpm. so that seems ok.
got it back on the snap on reader and traced the rpm graph - interesting at cranking doesn't show any rpm, then get a spike when it attempts to to start before flat lining again, then when it does start tracks perfectly. but could not find anything on live data for camshaft sensor?
Went on then to test the cable between crank sensor plug and ECU - eventually managed to pin out to 8th one down on row 1 and 7th one down on row 2, ran resistance test again - and consistent with previous reading 1180.
Tried to start with the crank sensor unplugged - strangely started which we weren't expecting, and much quicker 2 seconds of cranking - of course the engine light came on and no rev counter. Tried this a couple of times before plugging the crank sensor back on - back to starting after 5-10 seconds.
now have read on other posts which have suggested a software update has fixed this on other 500s, so likely to give the dealer a call explain the problem and find out how much to reflash to the latest engine software,
the saga continues....