Slarty, if you know how to operate a radio which you should do with a PPL, then get on the radio and call for help. Most transport cat. aircraft can do an autoland and it doesn't have to be at an airport certified for such.
To my understanding 2 such approaches were conducted in Australia long before there was any airport certified for them. There is now, Melbourne, with another 2 to follow, Perth and Sydney.
The biggest problem for a PPL holder is getting the beast set up for a landing which really means starting from the cruise altitude. The FMS will tell you when it is time to start down but if you don't set the Mode Control Panel to a lower altitude then the aircraft will cruise on happily until it runs out of fuel (think Helios). Then the STAR and runway needs to be programmed into the FMS (and the inevitable discontinuity removed from the LEGS page) then it needs to be slowed up and flaps and gear extended. Then you need to know how to set the autopilot for autoland.
None of it is difficult when you know how and it is possible to talk someone through the steps and what they have to do once the beastie is on the ground. However, throw in weather problems (say a crosswind and/or miserable vision etc) and it just keeps getting more difficult.
At the end of the day though, any pilot in such a situation should be prepared to give it a go rather than face the inevitable crash by not doing anything.