Andy:
Good question, however the answer is a little more difficult.
First lets examine an engine failure over the Channel vs. over the mountains.
If you ditch your chances of survival are dependant on two things. One: How close will your rescurers be? Two: Will you survive the landing?
Now with an engine failure over mountains, your chances of surviving the landing will be better as there is always an area where you can wreck the airplane and have a good chance of surviving. For one thing it is easier to breathe hanging from a tree than under water. Your chance of surviving a crash landing in mountains is in direct perportion to your flight planning. For example day vs. night. lots of altitude, good weather, follow roads or railroads whenever possible to name a few.
Risk management is up to the pilot, I do not fly single engine aircraft IFR, I do not fly single airplanes at night and I do not fly single engine airplanes over water beyond gliding distance from land.
Also I do not fly single pilot IFR, I just figure if the airplane has two or more engines and two or more radios it also needs two or more pilots.
Then that is only my personal risk management rules, you are free to form your own limits.
Remember one other factor, when you carry passengers you have a moral obligation not to unduly risk their well being.
Very good question Andy, just thought I would comment.
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The hardest thing about flying is knowing when to say no.