I then moved the box with the 3 dmm under test into my hot conservatory. Left them there for over 1 hour to acclimatise.
Then turned on the meters and measured again :
1905A 2.5V=2.4987V 5V=5.0039V 10V=9.9968V(control 26°C)
DMM under test, measured 37°C sorry no humidity measurement in conservatory.
UT61E 2.5V=2.499V 5V=5.005V 10V= 10.001V
U1232A 2.5V=2.498V 5V=5.004V 10V=9.99V
DT830B 2.5V=2.50V 5V=5.00V 10V=10.00V
Not sure what to conclude, yes the temperature did affect the reading but only by 3 counts worst case but that is within specification.
The Keysight shows no change what so ever, but then I would expect that.
The DT830 only chnages by 2 counts worst case. Amazing when you consider how cheap it is
(Sorry the patio door is very dirty
)