1. Est Mash efficiency @ 76.60% Vs Measured Mash efficiency @ 67.60%
You definitely need to calibrate your thermometer, hydrometer, and pH meter. The only other factor would be the mill crush if that could have changed.
I wouldn't worry about iodine; the test is almost entirely subjective and you can get more repeatable results by measuring gravity during the mash (I'm assuming the Grandfather recirculates, which should make that pretty easy).
2. Est Attenuation @ 67.90% Vs Measured Attenuation @ 59.40%
Because of the DME addition, I don't think you can conclude there was a problem here from a single data point.
I would never intentionally mash at those temps, especially with that O.G., unless I wanted to wind up with syrup which is about what you got.
In my experience, mashing at around 68°C yields normal attenuations (75-85% ADF depending on yeast strain). I didn't see any mention of the grist, so it could be that the base malt was under-modified and would have benefitted from a step mash - low conversion efficiency could support that, but again it's hard to conclude anything from a single data point.
Calibration & Grain Crush - Have calibrated, will double check though! Grain crush remains the same.
Iodine - Interesting as a lot of resources highly recommend it? I always check pre-boil gravity, will check gravity during next time though to see where I'm at.
Attenuation - I thought so too, but when I enter the DME addition into Beersmith the Est Attenuation is 67.90%?
Yeast - Wyeast 1728 Scottish Ale - Est 218 Billion
Grist for 10 Litre Batch:-
Marris Otter Pale (5.9 EBC / 2.80 L) = 4621.79 g (89.90 %)
Crystal Malt Light (100 EBC / 39 L) = 229.26 g (4.50 %)
Honey Malt (49.30 EBC / 19.03 L) = 119.56 g (2.30 %)
Chocolate Malt Pale (525 EBC / 197 L) = 58.55 g (1.10 %)
Crystal Malt Extra Dark (340 EBC / 128 L) = 58.55 g (1.10 %)
Munich Malt (25 EBC / 10 L) = 54.86 g (1.10 %)