Thanks Afmob for pointing out that my original step 13 wasn't necessary to reach naked singles.
Prelims
a) 12(2) cage at R1C1 = {39/48/57}, no 1,2,6
b) R1C34 = {69/78}
c) R12C7 = {39/48/57}, no 1,2,6
d) 12(2) cage at R1C9 = {39/48/57}, no 1,2,6
e) 12(2) cage at R2C3 = {39/48/57}, no 1,2,6
f) R3C56 = {39/48/57}, no 1,2,6
g) 12(2) cage at R4C5 = {39/48/57}, no 1,2,6
h) R78C1 = {39/48/57}, no 1,2,6
i) R7C78 = {49/58/67}, no 1,2,3
j) 15(2) cage at R8C2 = {69/78}
k) 12(2) cage at R8C8 = {39/48/57}, no 1,2,6
l) R9C34 = {39/48/57}, no 1,2,6
m) 12(4) cage at R2C4 = {1236/1245}, no 7,8,9
n) 12(4) cage at R2C6 = {1236/1245}, no 7,8,9
o) 12(4) cage at R5C5 = {1236/1245}, no 7,8,9
p) 12(4) cage at R6C2 = {1236/1245}, no 7,8,9
q) 12(4) cage at R8C6 = {1236/1245}, no 7,8,9
1. Hidden killer triple 7,8,9 in 12(2) cage at R1C9 and 15(2) cage at R8C2 for D/, 12(2) cage cannot contain more than one of 7,8,9 -> 15(2) cage must contain two of 7,8,9 = {78}, locked for N7 and D/, clean-up: no 4,5 in 12(2) cage at R1C9, no 4,5 in R78C1, no 4,5 in R9C4
1a. Naked pair {39} in 12(2) cage at R1C9 = {39}, locked for N3 and D/
1b. Naked pair {39} in R78C1, locked for C1 and N7, clean-up: no 3,9 in R2C2, no 3,9 in R9C4
1c. Naked pair {78} in R9C14, locked for R9, clean-up: no 4,5 in R8C8
[Interesting that I spotted the first step in a different way from Afmob, who saw it as hidden killer pair 7,9 for D/ eliminating 8 from the 12(2) cage, then 8 on D/ only in the 15(2) cage. That’s the technically simpler way to do it.]
2. Hidden killer triple 7,8,9 in 12(2) cage at R1C1, R7C7 and 12(2) cage at R8C8 for D\, 12(2) cage at R1C1 contains one of 7,8, 12(2) cage at R8C8 contains one of 7,8,9 -> R7C7 = {789}, clean-up: no 7,8,9 in R7C8
2a. 9 on D\ only in R7C7 + 12(2) cage at R8C8, locked for N9
2b. Hidden killer triple 7,8,9 in R7C7, R7C9 and 12(2) cage at R8C8 for N9, R7C7 = {789}, 12(2) cage contains one of 7,8,9 -> R7C9 = {78}
3. 45 rule on R5 3 innies R5C456 = 21 = {489/579/678}, no 1,2,3, clean-up: no 9 in R4C5
3a. R5C5 = {456} -> no 4,5,6 in R5C46, clean-up: no 7,8 in R4C5
3b. R5C46 + R6C5 = {789} (hidden triple in N5)
3c. 12(4) cage at R5C5 = {1236/1245}
3d. R5C5 = {456} -> no 6 in R6C67+R7C6
4. R5C456 = 21 = {489/579/678}
4a. R5C56 cannot total 12 which clashes with 12(2) cage at R4C5 (CCC) -> no 9 in R5C4
4b. Naked pair {78} in R59C4, locked for C4, clean-up: no 7,8 in R1C3
4c. Naked pair {69} in R1C34, locked for R1 -> R1C9 = 3, R2C8 = 9, clean-up: no 3 in R3C2
5. 9 on D\ only in R7C78 = [94] or in 12(2) cage at R8C8 = [39] -> 12(2) cage = [39/75] (cannot be [84], blocking cages), no 4,8
5a. 8 in N9 only in R7C79, locked for R7
[Step 5 could have been written as combined cages R7C78 and 12(2) cage at R8C8, which would have a lower rating, but I prefer the more elegant blocking cages.]
6. Hidden killer pair 7,8 in 12(3) cage at R7C5 and R9C4 for N8, R9C4 = {78} -> 12(3) cage must contain one of 7,8 = {138/147/237}, no 5,6,9
6a. R8C1 = 9 (hidden single in R8) -> R7C1 = 3
7. Hidden killer pair 1,2 in 12(3) cage at R1C5 and 12(3) cage at R7C5 for C5, 12(3) cage at R7C5 contains one of 1,2 -> 12(3) cage at R1C5 must contain one of 1,2 in C5 = {138/147/156/237/246} (cannot be {129} because 9 only in R3C4), no 9
7a. 1,2 must be in R12C5 -> no 1,2 in R3C4)
8. R5C456 (step 3) = {579/678} (cannot be {489} because R45C5 = [34] clashes with 12(3) cage at R7C5), no 4, 7 locked for R5 and N5
9. R5C456 (step 8) = {579/678} -> R5C456 + R6C5 = {579}8/{678}9 -> R56C5 = [58/69]
9a. Hidden killer pair 6,9 in R123C5 and R56C5 for C5, R56C5 contains both or neither of 6,9 -> R123C5 must contain both or neither of 6,9 but R123C5 cannot contain both of 6,9 (which clashes with R1C4) -> R56C5 = [69], 6 placed for both diagonals, clean-up: no 3 in R3C6, no 3 in R4C5
9b. Naked pair {78} in R5C46, locked for R5
9c. R5C5 = 6 -> 12(4) cage at R5C5 = {1236}, 3 locked for R6
[Cracked. The rest is fairly straightforward.]
10. 3 in N5 only in R4C4 + R6C6, locked for D\ -> R8C8 = 7, R9C9 = 5, both placed for D\, R8C2 = 8, R9C1 = 7, R2C2 = 4, placed for D\, R1C1 = 8, R9C34 = [48], R5C4 = 7, R5C6 = 8 -> R4C5 = 4, R7C9 = 8, R7C7 = 9 -> R7C8 = 4
11. R7C5 = 7 (hidden single in R7) -> R89C5 = 5 = {23}, locked for C5 and N8, R7C6 = 1, clean-up: no 5,9 in R3C6
11a. Naked pair {23} in R6C67, locked for R6
12. 12(4) cage at R6C2 = {1245} (only remaining combination) -> R7C3 = 2, placed for D/, R4C6 = 5, R6C4 = 1, placed for D/, R6C2 = 5, clean-up: no 7 in R2C3
13. Deleted
and the rest is naked singles, without using the diagonals.