Unfortunately, your radiology form needs to be stamped by the radiologist or hospital where you had your x-ray done. You also need a separate form from a GP to state that you are in good health. If you have the radiology report, you should be able to go back to the hospital or radiologist and ask them to sign and stamp it.
Did the radiologist give a reason why he didn't sign and stamp the original form? Keep in mind that the form needs to be filled in correctly (i.e. the empty spaces should be crossed out).