http://www.caa.co.uk/docs/33/CAP773.pdf
Overlay Approaches
An overlay approach is one that allows pilots to use GPS equipment to fly existing non-precision instrument approach procedures. However, many of these overlays do not accurately reflect the correct approach procedure and may even represent a different speed category of aircraft.
The normal equipment for that approach must always be used as the primary reference - not the GPS - otherwise any disparity between the displays and the potential for mistakes are just as likely to diminish the safety margins on approach as enhance them.
For example: VOR and NDB approaches to beacons actually on the destination aerodrome usually provide a final approach path or track which is not aligned with the main runway centre-line. Even on a direct approach to a particular runway, pilots should not necessarily expect to be on the extended centreline of the runway.
The terrestrial approach procedure may include DME ranges from the threshold, missed approach point or other reference, such as the beacon. The GPS may give distance guidance to a different point, such as the Aerodrome Reference Point. Pilots should be aware of any differences in the distance information given to step-down fixes and/or the MAP, as this has the potential for catastrophic error.