Anyway.... can anyone tell me definitively whether X channels are 63MHz higher or 63 MHz lower.
3. DME Signal Characteristics.
DME transmission frequencies are 1 MHz apart. There are 126 aircraft transmission channels running from channel 1 at 1025 MHz through channel 126 at 1150 MHz. There are two transmission modes for each channel. In mode X the ground station transmission frequency is 63 MHz below the aircraft transmission frequency for channels 1-63 and 63 MHz above the aircraft frequency for channels 64-126. In mode Y the frequency spacing is reversed, i.e., the ground frequencies for channels 1-63 and 64-126 are 63 MHz above and below the aircraft frequencies, respectively. Hence, in mode Y the ground station transmissions are in the air-to-ground band. To avoid confusion between transmissions from aircraft and ground stations on the same frequency and to discriminate pulses from random interference, pulses are always transmitted in pairs. From the aircraft in mode X the pulses are 12 microseconds apart, and in mode Y they are 36 microseconds apart. From the ground transmitter the pulses are 12 and 30 microseconds apart in modes X and Y, respectively. Mode X is used much more frequently than mode Y.
However, from the same paper. Your DME may not work unless you understand this. 
ABSTRACT
The neutral hydrogen 21-cm spectral line (1420.4 MHz) and the four 18-cm lines of the hydroxyl molecule (1612-1720 MHz) are observable at redshifts which put their measured line frequencies well below their protected frequency bands. Part of the redshift ranges (z = 0.171-0.477 for HI and z = 0.37-0.73 for OH) fall in the 960 to 1215 MHz band that is allocated to aircraft navigation. Most of the signals in this band are pulsed emissions of low duty cycle so much of the time between pulses is interference free. This paper outlines the structure and measured properties of signals in this band and demonstrates a signal processing strategy that is effective at removing the pulsed signals from spectra at sensitivities produced by several hours of integration.