TL/DR If you are going anywhere familiar or unfamiliar and it is unseasonably warm: take plenty of water, a mobile phone and charger, an analogue compass as well as a paper map if you have one.
Simon Calder, Travel Correspondent for The Independent after being asked if Mosley should had taken a mobile phone and 'what are the main things one should take with one in that sort of heat'
"Can I just say that I am absolutely in tune with how he was feeling and the decision that he made. It might be seen as irresponsible and of course with hindsight we can all look back and say yes, of course he should have had his phone with him, um, yes he should have made sure he had plenty of water and ideally he should have been, um, hiking with somebody else since as he was - as his wife says , he was taking a really challenging hike....."No, Mr Calder! It's not just with 'the benefit of hindsight'. It was irresponsible at the time. The locals don't go out 'hiking' in the midday sun and nor do 'hikers' go out in 40°C/100°F+ heat if there is a choice not to. Michael Mosley's wife, Dr Clare Bailey Mosley a GP until 2022 also said - when he was still just 'missing' - that Mosley was feeling ill and decided to go home'. Is that the advice a GP would normally give? Feeling ill? Why not walk home in difficult terrain in 40 degree heat. No, don't bother taking any water with you, an umbrella should do it.
"... we can say ultimately it's a beautiful island, ...however, he was the sort of person who was very very good at understanding risks at knowing the issues that, um, he could possibly face. And I can just see that he...it's a beautiful day...Yes it's hot, but he's confident in his own abilities because he's shown time and time again that, er, he's, he's a..a great survivor. So I can see that he would set off like this. I think ALL of us, um, looking at the terrain that there is, the extreme heat which currently is affecting Symi and other parts of the Easter Mediterranean you would I think now think again and, and consider, um, 'am I properly equipped have I got all I need in case there is a fall, or I feel weak or whatever', but ultimately it's a beautiful island , it's was a fine day...he felt that he wasn't far from safety... it was just I'm so sorry to say - misadventure -something went tragically wrong, and, it has ended with the loss of a great husband, a great father and a great communicator who helped us all, um, improve our health."Not it's not just 'hot'. It was unbearably hot according to a BBC journalist reporting on this in situ.
(A couple of days ago I wrote a piece about the bizarre episode of Dr Michael Mosley's disappearance. It was somewhat tongue-in-cheek in parts and I took the odd liberty to make a point but I stand by what I wrote then even more so now...I am ex-military; soldiers die regular on SAS selection from heat exhaustion in temperatures on Pen y Fan nothing like 40 degrees. In fact the Training Staff would not put people out in temperatures such as that.) Mosley and his wife, both medical doctors , would have been well aware of the the importance of hydration. The friend she was staying with would have been well aware of the dangers of the island, the trecherous rocky terrain