As as ATCO at a non-radar unit in the uk, I feel it might be worthwhile detailing events which happened some months ago on this very subject. A/c inbound to above unit makes contact, IMC & requesting radar vectors to final approach. Pilot advised that radar is unavailable, and is asked if he is able to effect an approach procedure to land. Pilot responds, 'Looks like I'll have no choice because I'm flying on GPS but it's giving erroneous readings.' He also stated that his nav kit both NDB and VOR weren't functioning 'correctly'. Pilot is instructed to climb to MSA and report overhead the XXX beacon. After a lot of protracted R/T, I became very aware that this pilot was hopelessly lost and utterly dependent on his 'erroneous' GPS. This particular pilot was almost pleading for radar assistance at an airfield which was totally unable to provide it. After 30mins of dialogue between pilot, D&D and other nearby radar units, (although not near enough to give vectors AND descent) the a/c was identified at 800ft in an area of high terrain of 2400ft in IMC. Ultimately the pilot was lucky enough to get enough surface contact to effect a landing. After landing, his nav kit was checked and found to be perfectly serviceable but simply turned off. This pilot did not know how to navigate on standard instruments and was basically a GPS pilot. When the GPS failed, he was lucky to escape disaster. Be warned!!!