You're right Nige, about the vapour lock situation.
It is 20 deg C that you have to remember ( not 25 deg C )
But it is not the OAT thats important , it is the temperature of the fuel you need to be aware of................ a dark coloured tank sitting in the sun all day can absorb a great deal of heat, which results in a fuel temperature significantly higher than the OAT.......So a bright sunny day with temp hovering around, say +18 could result in your tank temperature of + 24 or more.
Conversely , you can use freshly obtained forecourt fuel on a +30 deg C day which because it comes from underground tanks is quite a bit cooler.....around +12 ( when I've checked it ), so long as you uplift it and use it before it has a chance of warming up significantly........
(Yes, I know that north of Watford ,plus 30 days are unheard of
, so it wont be a concern ! )
The temperature within the tank could also vary ........near the top with the warmed up metal exposed to the sun is always going to be more than the lower surfaces where fuel is drawn from
The LAA have withdrawn their info about all this due to the Alcohol / Ethanol issue but I think it was almost a direct copy of the Airworthiness Notice 98 which is still available from the CAA. This is applicable for the poor souls who operate on a C. of A .
AWN 98A listed the limitataions, and AWN98 C ammended these for the use of Unleaded.
There is also the Pressure altitude limit of 6000 feet which again is a attempt to prevent occurances vapour locking.
Both limits are pretty conservative and probably a result of "one size fits all " approach .
rgds Phil ( currently looking at Solar hot water heating systems for the house ! )