I have an image of license plate in black and white.
this is how it looks:
Assuming you have the box surrounding the letters - which gives you the overall angle
Collapse the image down into 1d (may help to rotate it first so the bounding box is horizontal)
Then look for the gaps between letters in this 1d signature giving you the positions of the digits. It helps if you know the number of digits and the format for the plates.