Thanks both for suggestions - the image is a giant XXL one at 110"x87.5" so will be printed as TWO long strips of 110x44" paper, half the image on each.
The joint between the two halves is the issue, as a white strip is not perfect - trimming off a 110 x 1/8" white border is hard to do perfectly - hence the original question.
Ideally for the joint between them, one side would be printed borderless, easier to join up side by side, and no white strip down the middle.
Will try simply adjusting the printer centering on one side and see if that makes it 'borderless'.