Which ruined abbey ?
It's not really dark even at 10pm at the moment, its gonna be a long shutter whatever but the conditions aren't quite the same as in November.
I'd take a few shots of the sky pre-fireworks and see what the longest shutter time is before the sky starts getting too bright.
It won't hurt to have a dark blue backdrop though instead of pitch black, could be cool..
god, I just looked out the window, its not even slightly dark, what time is the display cos shutter speeds aren't going to be that long at all..