Have searched the forums and the net for an answer to my problem, (apologies in avance if it has already been answered), which on the face of it is quite simple, that of "no service" I have however tried every thing I and others can think of....

First of all symptoms:

Started a few weeks ago and has gradually got worse, phone is out of warranty.
Phone toggles between "searching" and " no service". With 3g on!
When 3g is turned off, some of the time it shows one bar and occasionaly for a few seconds shows three bars and the network, but this rapidly disappears!

Have checked:

All the normal things, reset network settings, restore etc., etc., have tried sim in another phone,all ok....checked network provider (o2)to ensure not barred (unlikley).Checked and cleaned sim contacts, ensured copper aerial is contacting frame.
Have swapped logic board to ensure aerials and rest of hardware are working ok.
Have jalbroken, unlocked,with ultrasnow and upgraded baseband to 6.15. tried 3.03/4.1/4.2.

Conclusions.

Not hardware (exception of logic board) not baseband, not os,not sim, not network,

Thanks for reading what is rather a long post

os 4.2
baseband 6.15