First off, @Ideasmiths, you only need to hold down both buttons for 7 seconds before the iPad will reset. Then simply let go and press the power button, which will restart the iPad.
It is a common misconception that you need to hold them for a long time, like +2 minutes. There is no truth in that. The reset mechanism is built into the hardware of the iPad and cannot be changed and cannot hang or fail. It is set at just over 6 seconds. What sometimes happens is your iPad resets and stays off, which is why you think a long press is required. All you have to do is hold them both for 7 seconds. Let go of both. Then press the power button (which will turn it on). This will always restart the iPad 100% of the time. No need to wait.
@alontowing, if you have performed a reset as you say and the Apple logo sticks and the iPad never boots then it sounds like you have a corrupted file system. This is very rare, but does happen. Do you know what app you were updating? There has been no Cydia system update for a long time, so I am assuming it must have been an application...
Do you have OpenSSH installed? If so, when the iPad is hanging at the Apple logo, are you able to SSH onto the device from your PC? If yes, you may be able to uninstall the App which you were updating (if you can remember what it was). If so, let us know.
Otherwise, as long as you have given it a good chance to reboot (it can take several minutes if there has been a problem), then you are probably out of luck and will have to restore to 4.3.5.
I would leave it at the Apple logo for at least 5 minutes. Don't be tempted to re-set over and over. If it has still not rebooted after that then it's restore time I'm afraid
