Very grateful for your comments.
I am very much a beginner and even simple things can elude me time to time. My apopogies for not seeing the obvious. In this case, it seems that by moving to to a new and unused part of the breadboard (unused) I tightened up the connections - and then everything worked! So, now I get this from my code:
"Temp0 = 21.50 Temp1 = 21.00Hi!
Temp0 = 21.50 Temp1 = 21.00Hi!
Temp0 = 22.00 Temp1 = 23.00Hi!
Temp0 = 22.50 Temp1 = 24.50Hi!"
And with a modified code, even managed to get addresses:
"Requesting temperatures...DONE
2.46
Device Address: 28B49BA90500000D Temp C: 21.00 Temp F: 69.80
Device Address: 28A19DA905000021 Temp C: 20.50 Temp F: 68.90
Device Address: 28C3CAA8050000C7 Temp C: 21.00 Temp F: 69.80
Requesting temperatures...DONE"
Someone needed to point out the obvious!
Many thanks,
Colin.