So to keep things short and sweet, but still give enough detail:
Phone: SGS2 i9100
Flashed the correct Siyah kernal - ran it for a few days with the stock rom and all was good
Decided to flash a rom and after reading about Neatrom I followed the instructions on Sale's Neatrom page on XDA...
.... all seemed to go OK. Flashed the custom recovery, flashed the rom, flashed the gapps and rebooted.
All seemed to be OK until it fully booted - to the screen where it asked for my pin. I wasn't expecting this, but used the pin I would normally use - didn't work. Tried a few obvious default style pins like 1234, 1111 etc. Nothing worked.
Decided to boot into recovery and flash the nandroid backup I had made, but instead of booting up into CWM it booted in TWRP and I could find the nandroid backup.
Perplexed I tried reflashing the rom - this seemed to go OK, but then when I tried to reboot, the phone got stuck on the Neatrom splash screen.
I feel like an idiot (I guess I am!)
Many thanks for reading and I hope someone is gracious enough to help me out.