I had only just started A81 when Afmob, in a message about another walkthrough, commented that it's easier using innies/outies and asked what I thought about the posted ratings?
Since I'd only done Prelims and about 15 steps I had another look at innies/outies and managed to find the key one which I've inserted as step 10. Thanks Afmob.

It definitely makes the solution quicker and easier. I therefore rate A81 as 1.0.
Here is my walkthrough
Prelims
a) R23C1 = {39/48/57}, no 1,2,6
b) R45C4 = {17/26/35}, no 4,8,9
c) R78C1 = {59/68}
d) R78C5 = {16/25/34}, no 7,8,9
e) R78C9 = {19/28/37/46}, no 5
f) R23C9 = {13}, locked for C9 and N3, clean-up: no 7,9 in R78C9
g) R5C123 = {128/137/146/236/245}, no 9
h) R5C789 = {128/137/146/236/245}, no 9
i) 22(3) cage at R6C1 = 9{58/67}, CPE no 9 in R4C2
j) 12(4) cage in N1 = {1236/1245}, 1,2 locked for N1
k) 28(4) cage in N3 = {4789/5689}, 8,9 locked for N3
l) 14(4) cage at R6C3 = {1238/1247/1256/1346/2345}, no 9
1. 45 rule on R5 3 innies R5C456 = 23 = {689} -> R5C6 = 6, R4C6 = 2, R5C45 = {89}, locked for R5 and N5
1a. 30(5) cage in N5 = {15789} (only remaining combination), locked for N5
1b. 45 rule on R6789 2 innies R6C56 = 6 = {15}, locked for R6 and N5 -> R4C4 = 7
1c. R6C89 = [34] clashes with R6C4 -> no 8 in R7C8
1d. R5C123 = {137/245}
1e. R5C789 = {137/245}
2. R78C5 = {16/25} (cannot be {34} which clashes with R4C5), no 3,4
3. 45 rule on C6789 1 outie R9C5 = 1 innie R6C6 + 1, R9C5 = {26}
4. Naked quad {1256} in R6789C5, locked for C5
4a. Killer pair 2,6 in R78C5 and R9C5, locked for N8
4b. 7 in C5 locked in R123C5, locked for N2
5. R234C5 = {347} (only remaining combination), locked for C5
6. 22(3) cage at R6C1 = 9{58/67}
6a. 5 of {589} must be in R7C2 -> no 8 in R7C2
7. 45 rule on N1 3 innies R1C3 + R3C23 = 21 = {489/579/567}, no 3
8. 45 rule on R12 3 innies R2C159 = 14 = {149/347} (cannot be {158} because 5,8 only in R2C1), no 5,8, 4 locked for R2, clean-up: no 4,7 in R3C1
8a. 3 of {347} must be in R2C9 -> no 3 in R2C15, clean-up: no 9 in R3C1
9. 45 rule on R89 3 innies R8C159 = 16 = {169/259/268}, no 4, clean-up: no 6 in R7C9
9a. 9 of {259} must be in R8C1 -> no 5 in R8C1, clean-up: no 9 in R7C1
10. 45 rule on R12 2 outies R3C19 = 1 innie R2C5 + 2
10a. R2C5 = {47} -> R3C19 = 6,9 = [51/81] (cannot be [33]) -> R3C1 = {58}, R3C9 = 1, R2C9 = 3, clean-up: no 9 in R2C1
10b. Naked pair {47} in R2C15, locked for R2
11. 3 in N1 locked in R1C12, locked for R1
11a. 12(4) cage in N1 = {1236}, locked for N1
12. Killer pair 5,8 in R3C1 and R78C1, locked for C1
13. 45 rule on N7 3 innies R7C23 + R9C3 = 12 = {129/147/237/246/345} (cannot be {138} because no 1,3,8 in R7C2, cannot be {156} which clashes with R78C1), no 8
13a. 5,6,7,9 must be in R7C2 -> no 5,6,7,9 in R79C3
14. 15(3) cage at R8C4 = {159/249/258/348}
14a. 1 of {159} must be in R9C3 -> no 1 in R89C4
15. Killer pair 8,9 in R5C4 and R89C4, locked for C4
16. 14(3) cage at R3C2 = {149/158/167/347/356}
16a. 8 of {158} must be in R4C12 (cannot be [815] which clashes with R5C123)
16b. 9 of {149} must be in R4C1 (cannot be 9{14} which clashes with R5C123)
16c. -> no 8,9 in R3C2
16d. 5 of {158/356} must be in R3C2 -> no 5 in R4C2
17. R1C3 + R3C23 (step 7) = {489/579/567}
17a. 4 of {489} must be in R3C2 -> no 4 in R13C3
18. 14(3) cage at R1C6 = {149/158/248} (cannot be {167/257} because 2,6,7 only in R1C7), no 6,7
18a. 5 of {158} must be in R1C7 -> no 5 in R12C6
18a. Killer pair 8,9 in R1C5 and R12C6, locked for N2
19. 8,9 in R3 locked in R3C13 -> R3C1 = 8, R3C3 = 9, R2C1 = 4, R2C5 = 7, clean-up: no 6 in R78C1 -> R78C1 = [59], clean-up: no 2 in R8C5
20. R6C2 = 9 (hidden single in C2)
20a. R6C1 + R7C2 = {67}, CPE no 6,7 in R45C2
21. R8C59 (step 9) = {16/25}, no 8, clean-up: no 2 in R7C9
21a. 6 of {16} must be in R8C9 -> no 6 in R8C5, clean-up: no 1 in R7C5
22. R3C3 = 9 -> R3C4 + R4C3 = 9 = {36/45}, no 1,2,8
22a. 2 in R3 locked in R3C78, locked for N3
23. 14(3) cage at R1C6 (step 18) = {149/158}, 1 locked in R12C6, locked for C6 and N2 -> R6C56 = [15], R8C5 = 5, R7C5 = 2, R9C5 = 6
23a. 4 of {149} must be in R1C7 -> no 4 in R1C6
23b. R7C4 = 1 (hidden single in C4)
24. Naked pair {34} in R3C56, locked for R3 and N2, clean-up: no 5,6 in R4C3 (step 22)
24a. Naked pair {34} in R4C35, locked for R4
25. 14(4) cage at R6C3 = {1238/1247/1346}
25a. 6,7,8 must be in R6C3 -> R6C3 = {678}
26. 2 in N5 locked in R5C123 = {245} (step 1d) -> R5C1 = 2, R5C23 = {45}, locked for R5 and N4 -> R5C9 = 7, R4C3 = 3, R3C4 = 6 (step 22), R34C5 = [34], R3C6 = 4, R6C4 = 3, R7C3 = 4, R6C3 = 6 (step 25), R6C1 = 7, R7C2 = 6, R5C23 = [45], R1C3 = 7, R3C2 = 5, R78C9 = [82], R6C9 = 4
26a. Naked pair {13} in R5C78, locked for N6
26b. Naked pair {28} in R6C78, locked for N6
27. Naked triple {569} in R4C789, locked for R4 -> R4C1 = 1, R4C2 = 8, R9C1 = 3, R1C1 = 6
28. R8C3 = 8 (hidden single in C3), R8C4 = 4
29. Naked pair {12} in R2C23, locked for R2 and N1 -> R1C2 = 3, R12C4 = [25], R1C5 = 8 {cage sum), R12C6 = [19], R1C7 = 4 (step 23), R5C45 = [89], R9C4 = 9, R9C3 = 2 (step 14), R2C23 = [21], R9C9 = 5, R1C89 = [59], R4C89 = [96], R4C7 = 5
30. R89C6 cannot be {78} -> R789C6 = [738]
and the rest is naked singles and cage sums