mobile

I have a Samsung Galaxy tab 2 GT-P5110 which I had successfully installed CM12 onto. I decided to set up work email which insisted that I encrypt the device. I believe that the process completed successfully.
At that point I decided that I would like to flash the device with CM12.1. Restarted the process (Factory reset, Wipe Cache, Wipe dalvik cache) and flash the new CM12.1 zip file from external SD card.
I cannot now get to the recovery mode and the device doesn't always boot into anything recogniseable. The screen goes black and blacker! I have manaed to get to the CM logo and then a screen asking me for the PIN for the encryption. The next screen tells me that the PIN is right but the data is corrupt. It gives me the option to "RESET TABLET" but that really doesn't do anything and won't let me in to the recovery menu.
I can get into the Download mode and connect Heimdall up to it.
Is there anything that anyone can point me at that will help me to unbrick it?
I hope I've passed on enough information to help. It seems to me I need to somehow re-format the internal SD but I can't get to any menu that will allow me to do that.
Any thoughts anyone?
Ta