Hi.. I tried a new battery, that's what caused the boot loop (3600mah)..
I fixed this on this phone.. Maybe entirely different cause for some one else..
I noticed a power drain last night after swapping the gutt's to a new shell.. Everything worked fine, so presumed the battery was shot.. So changed it for a high powered battery, which seemed to work fine.. Was about to call the client to say its fixed, one last test.. Boot loop!!
Any way.. After searching lots of posts like this one decided to check for shorts.. Took every thing back out, tested the board while it was unscrewed, but still in place.. It worked!
So screwed it back together testing the boot sequence with every few screws.. The very last connector.. Front facing camera flex! Boot loop.. If I unplug it while in boot loop it works fine and boots up.. So I'm just waiting for a replacement flex and preying the fault is the flex and not on the logic board.. Fingers crossed