kirkwoodill
Lung Function Tests are notoriously difficult to do well. You should have had at least three goes with the machine at the testing centre and only results that are close to each other should be used.
The Peak Flow Meter is not a proper Spirometer and gives different numbers. The two are not interchangeable (although Peak Flow is often used as a crude measure of lung function as it is easy and cheap to do).
It will be interesting to hear what your AME says. The first thing to check is whether you blew 57% of predicted (not great but would be better with correct technique) or whether your FEV1 (Forced Expiratory Volume in one second) is 57% of your FVC (Forced Vital Capacity). Both are measured at the same time. FVC is all the air you blow out in that long sustained expiration. FEV1 is the value on that curve at 1 second. The ratio between one and the other defines the difference between restrictive and obstructive lung disease (its more complicated than that but this is not a text on Respiratory Physiology!).
If your AME says he is refusing a licence on the grounds of the Lung Function Tests then I would ask for a retest on the grounds that you do not think you performed the tests as well as you could have. If he passes you then I wouldn't worry. The tests might have been more accurate than you thought!
HTH
BtD