What is suppling the 12v?
Looks like you are trying a cct. similar to above i.e. Q1 where Q2 would would apply.
If you have the 102AX, it is common cathode.
Large digits are usually driven with a single TPIC6B595 chip (or family member).They can be daisy-chained for e.g. a whole score board.And that would only use three Arduino pins.
I have just checked the TPIC6B595, they appear to be a good solution for my problem, but before I order any, I need to be sure that they can drive my digit (only 1 negative pin)
Here is the circuit schematic:
Do you have:102AXor102BX
If you want to make things easier then throw them in the bin and order new common anode displays.
With common cathode you require PNP or p-MOSFETs to switch the 12V high-side, and a small-signal NPN transistor as a level shifter. The BC337 could be an NPN level shifter.See post #4 in this thread: https://forum.arduino.cc/index.php?topic=555862.0