Hi Batang,
Thanks for your feedback - turns out it was the sim connections. Farnell 2291185 - they number 1,5,2,6,3,7,4,8 not 1,2,3,4,5,6,7,8 as I did - what a muppet!
We have changed the SimVDD cap to .22uf, as for PSU we have used the recommened SMPS.
Have ordered a breakout board to solder the connections for sim card again, and will give a final report on this next week - fingers crossed.
I think may others with the same fault (plenty on this forum) really do need to check connectivity - simple mistake causes the exact issues reported by others.
Cheers
Matt