In reply to JoshOvki:
I don't think its grey... every policy I have had in 24 years whether 3d party or comp has stated either nothing or (more usually) that I can drive other cars not owned by me that are not hire cars on a 3d party basis.
The only bit that isn't spelt out but which seems pretty obvious and well known is that it is not legal to drive a car or have it on public land unless it has its own policy attached to it. So that means that when driving a veicle using teh 'not owned by me 3d party basis' that vehicle still needs to have a valid policy on it as well.
So - one always has a policy that either is silent on the matter (you can't) or pretty clearly states that you can.... and the vehicle in question will have its own policy that covers its owners and drivers and covers it for use on an open road.
For the OP - if willing to accept risk of a 3d party crashing your car simply ask to see their policy and if it says they can do it and your car still has a policy on it, allow them to test drive with you in the passanger seat and the keys only handed to them when you are both in the car and handed back to you at end of th edrive before you get out! If not willing to take risk on them crashing your car the only route to go down is either you test drive with them in passanger seat or they produce a cover note from a day insurance place for the car on the correct day. Or you do that together on-line in your house before going out for a drive (hopefully at their expense). If somebody is serious a viewing, you doing a drive to show that car is OK and makes no nasty noises and then them arranging cover so they can drive would be reasonable.