Thanks Afmob for your comments and corrections.
Prelims
a) R1C45 = {15/24}
b) R23C5 = {49/58/67}, no 1,2,3
c) R23C9 = {39/48/57}, no 1,2,6
d) R45C1 = {89}
e) R4C89 = {79}
f) R5C23 = {12}
g) R89C4 = {19/28/37/46}, no 5
h) R9C23 = {17/26/35}, no 4,8,9
i) 9(3) cage at R1C6 = {126/135/234}, no 7,8,9
j) 8(3) cage at R7C7 = {125/134}
k) 26(4) cage at R6C3 = {2789/3689/4589/4679/5678}, no 1
l) 27(4) cage at R8C8 = {3789/4689/5679}, no 1,2
m) 23(6) cage at R4C7 = {123458/123467}, no 9
Steps directly and indirectly resulting from Prelims
1a. Naked pair {79} in R4C89, locked for R4 and N6 -> R45C1 = [89]
1b. Naked pair {12} in R5C23, locked for R5 and N4
1c. 8(3) cage at R7C7 = {125/134}, 1 locked for N9
1d. 27(4) cage at R8C8 = {3789/4689/5679}, 9 locked for N9
1e. 7 in N4 only in R6C123, locked for R6
1f. 23(6) cage at R4C7 = {123458/123467}, 1 locked for R6 and N6
1g. 7 of {123467} must be in R7C9 -> no 6 in R7C9
1h. 9 in C7 only in R13C7, locked for N3, clean-up: no 3 in R23C9
2. 45 rule on R123 2 innies R23C4 = 1 outie R4C7 + 11
2a. Min R4C7 = 2 -> min R23C4 = 13, no 1,2,3 in R23C4
2b. 3 in N2 only in R123C6, locked for C6
3. 45 rule on N7 3 innies R7C23 + R8C3 = 21 = {489/579/678}, no 1,2,3
4. 45 rule on N9 2 innies R7C9 + R9C7 = 10 = {28/37}/[46], no 5, no 4 in R9C7
5. 45 rule on C123 2 innies R4C23 = 1 outies R7C4 + 4
5a. Min R4C23 = 7 -> min R7C4 = 3
5b. Max R4C23 = 11 -> max R7C7 = 7
6. 45 rule on C1234 3 innies R156C4 = 12
6a. Min R15C4 = 4 -> max R6C4 = 8
6b. 9 in R6 only in R6C56, locked for 31(5) cage at R5C6, no 9 in R7C6
7. 45 rule on R1234 2 remaining innies R4C56 = 5 = {14}/[32], no 2 in R4C5
7a. 45 rule on R1234 3 remaining outies R5C45 + R6C5 = 18 = {378/468/567}, no 2
7b. R4C23 = R7C4 + 4 (step 5)
7c. Min R4C23 = 8 (R4C23 cannot be {34} which clashes with R4C56) -> min R7C4 = 4
8. 45 rule on N7 2 outies R6C3 + R7C4 = 1 innie R7C2 + 5, IOU no 5 in R6C3
8a. Max R6C3 + R7C4 = 13 -> max R7C2 = 8
9. 45 rule on C1234 4(1+3) outies R1C5 + R4C56 + R5C5 = 17, R4C56 (step 7) = 5 -> R15C5 = 12 = [48/57], clean-up: no 4,5 in R1C4
9a. R23C5 = {49/67} (cannot be {58} which clashes with R15C5), no 5,8
9b. Killer pair 4,7 in R15C5 and R23C5, locked for C5, clean-up: no 1 in R4C6 (step 7)
10. R7C9 + R9C7 (step 4) = {28/37}/[46]
10a. 45 rule on N6 2 innies R46C7 = 1 outie R7C9 + 6, R7C9 = {23478} -> R46C7 = 8,9,10,13,14 = {26/36}/[58/68] (cannot be {46} which clashes with R7C9 + R9C7 = [46], killer combo clash, other combinations eliminated because R46C7 must contain 6 except when R7C9 = 7), no 4 in R4C7, no 4,5 in R6C7, no 4 in R7C9, clean-up: no 6 in R9C7
10b. Killer pair 2,3 in 8(3) cage at R7C7 and R7C9 + R9C7, locked for N9
11. 45 rule on N5 2 outies R6C7 + R7C6 = 1 innie R4C4 + 9, min R6C7 + R7C6 = 10, max R6C7 = 8 -> min R7C6 = 2
12. 45 rule on N3 2 outies R3C6 + R4C7 = 1 innie R2C7 + 6, IOU no 6 in R3C6
[I ought to have seen this 45 earlier]
13. 45 rule on N3 3 innies R2C7 + R3C78 = 10 = {127/136/235} (cannot be {145} which clashes with R23C9), no 4,8,9
13a. 16(4) cage at R3C6 = {1267/1357/1456/2347/2356} (cannot be {1249/1348} because 4,8,9 only in R3C6, cannot be {1258} because no 4,7 in R2C7), no 8,9 in R3C6
13b. R1C7 = 9 (hidden single in C7)
14. 8 in N2 only in R23C4, locked for C4, clean-up: no 2 in R89C4
14a. R23C4 = R4C7 + 11 (step 2) -> R23C4 = {58/68/89}, R4C7 = {236}
14b. 27(5) cage at R2C4 contains 8 = {14589/23589} (cannot be {13689} which clashes with R4C5) -> R23C4 = {89}, locked for C4 and N2, R4C7 = 6, R4C4 = {12}, clean-up: no 4 in R23C5, no 1 in R89C4
14c. Naked pair {67} in R23C5, locked for C5 and N2 -> R5C5 = 8, placed for both diagonals, R1C5 = 4 (step 9), R1C4 = 2, R4C4 = 1, placed for D\, R4C5 = 3, R4C6 = 2 (step 7), placed for D/
14d. R4C56 = [32], R5C5 = 8 -> R56C4 = 10 = {46}, locked for C4 and N5
14e. Naked triple {135} in R123C6, locked for C6 -> R5C6 = 7, R6C6 = 9, placed for D\, R6C5 = 5 -> R6C7 + R7C6 = 10 -> R6C7 = 2, R7C6 = 8
14f. R9C7 = 8 (hidden single in C7), R7C9 = 2 (step 4)
14g. 8(3) cage at R7C7 = {134} (only remaining combination), 4 locked for N9
14h. Naked triple {345} in R5C789, locked for R5 and N6 -> R5C4 = 6, R6C4 = 4, placed for D/
15. R3C7 = 7 (hidden single in C7), placed for D/, clean-up: no 5 in R23C9
15a. R2C7 + R3C78 (step 13) = {127} (only remaining combination) -> R2C7 = 1, R3C8 = 2, R3C6 = 1 (cage sum)
15b. Naked pair {34} in R78C7, locked for C7 -> R5C7 = 5, R7C8 = 1, R6C89 = [81], R7C5 = 9
16. R7C4 = 5 (hidden single in C4), R7C3 = 6, placed for D/, R68C3 = 15 = {78} -> R6C3 = 7, R8C3 = 8
16a. R6C12 = {36} -> R7C2 = 7 (cage sum)
16b. R9C23 = {35} (only remaining combination), locked for R9 and N7 -> R7C1 = 4, R7C7 = 3, placed for D\, R9C4 = 7, R9C9 = 6, placed for D\
16c. Naked pair {57} in R1C1 + R8C8, locked for D\ -> R3C3 = 4
and the rest is naked singles, without using the diagonals.
Afmob pointed out that step 16c isn't needed to reach naked singles without using the diagonals. However the finish is a lot quicker if step 16c is used.