Yep! I assumed that the axle was symmetrical. which is wrong. The extra length of the BB axle is not split equally between left and right, but is not 100% on the right side.
I tried to find out the exact specs of the UN55 BB, but I failed. Even
Sheldon Brown is rather vague on this point.
The BB is available in 122.5 and 127.5 mm. Since they cost only £10 each, I think you can afford trying both sizes!