As already mentioned, an ELT / mobile phone / forewarned destination is probablyl a lot more use than a flight plan. With unpredictable weather over the mountains, your flight path is not going to be very predictable either - so if you don't show up at your destination the routing you marked on the flight plan has a good chance of being significantly different from your actual flight path.
Whirlybird
I asked for advice in advance, and was told that in the event of engine failure I should head for the valleys;
The problem with that is that the valleys are likely to have small fields surrounded by hedges or walls. An engine failure is likely to end in a nasty accident. Flying in the Alps, the advice is always to head for open ground above the treeline, and land uphill. You have a better chance of avoiding a high-speed collision with something hard. The downside, of course, is that if you do hurt yourself you are further away from help. (Other downside is that you will probably need to pay for a helicopter to collect what's left of your aeroplane.....)