Does this occur when you have not used the car for a while, or does it occur when you have made a short trip to the filling station, for example, and the car has not fully warmed up? When you return after paying, you find it will not start?
It could be something elusive like mikeloadsasaabs issue, or it could be one of the more common automatic transmission Saab problems, such as the neutral safety switch (NSS) (also called range switch) or the crank position sensor.
If it occurs when the car is stone cold, I would say it is probably not the NSS as that seems to be a failure that affects Saabs after short trips and resolves when the engine cools below a certain point.
If it is the NSS, there are various things on Saab websites describing people wiring switches in to bypass it (I am not sure that's a great idea), to pouring water on the switch to cool it when the fault occurs and then the car will start. That can be an inexpensive way of excluding or confirming a diagnosis.
Ultimately, if it is the NSS it will need replacing. It cost about A$500 over here but I imagine it may be cheaper in the UK.
Good luck, be sure to let us know what the outcome is.
