Won't switch on or charge, no power at all?

Changed digitiser in an iPad 2 and customer took it away yesterday, all good. Called today to say it wouldn't power on.

Brought it back in and it's dead, won't turn on, no charge indication, nada. No communication with desktop when hooked up via USb either so it's not an LCD issue.

So I've opened it up and I'm at a loss here.

I've swapped the logic board into another unit and it works fine so it's not that. Changed dock connectors, checked battery voltages at the battery and it appears to be charged.

Can't be the power button as it would boot when connected to external power and it doesn't.

I can't think what else to change. Could it be some dodgy battery issue, not easy to swap a new one in to test but it's about the only thing I haven't swapped in, and the logic board works fine in another unit.

UPDATE

It is turning on, I can connect to it via iTunes, but there is no display. I can see the panel light up, from the back you can see it lit, but from the front you just get a very feint indication that the panel is powered on, no display. It's not the same as the reset issue where no backlight but display works.

I've changed the LCD panel, and the cable, same issue persists :(