Chaps, I think some peeps have missed the point of Mighty Gems thread - a published approach to an airfield is safe because a) everyone knows where the aircraft will be because the approach direction is widely promulgated and b) has a proper approach minima based on obstructions on the approach path. An ad hoc GPS approach has neither attribute and despite the accuracy of GPS, it is downright dangerous to fly this type of non published letdown in congested airspace.
In an emergency or if you have got completely caught out by the weather with no fuel to divert, then you might have a valid case - but if you plan to do this type of letdown on a routine basis you should hand in your license an IR and go and do something else for a job.
If anyone out there is doing this type of letdown in UK then I hope it is not me you kill when you either have a mid-air or collide with a mast that you didn't know about.