How to solve The Times' most difficult Killer Sudoku

Simon explains how to solve this Deadly-rated puzzle from 3 February.

[Andyuk]

Amazing to follow your thought process. Great work!

[nanwarren]

Really enjoyed seeing that it is ok to pencil in the possible numbers; it is ok not to be able to keep them in your head! The only problem is ads interfering-both interrupting and covering. I missed some steps because of them. But thank you!

[richardfarrer]

Lucky escape at 16:00 when you wrongly worked out that 5 + 9 + *4* = 19. You could have excluded 5 + 9 + 5 instead to show it must be 7 + 9 + 3. Fortunately you derived the 3 separately anyway

[Normal_Average_Guy]

Can I purchase this program from somewhere?