Thanks so much Anton! I’ll try that out ASAP and let you know how the results compare.
I also had support from Walter Volpatto over on Facebook and he suggested the following:
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In Project Settings, set the project to “ACEScct” and ACES Output Device transform to “Rec.709”
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Create three nodes on the “Color” page
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Add a Color Space Transform OFX to the first node and set it to:
- Input Color Space - ACES (AP1)
- Input Gamma - ACEScct
- Output Color Space - ARRI Alexa
- Output Gamma - ARRI LogC
- no “Tone Mapping”, no “Gamut Mapping”, no ticks under the “Advanced” options.
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Apply the ARRI Alexa LUT / Phantom LUT to the second node
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Add an ACES Transform OFX to the third node and set it to:
- Input Transform - Rec.709 (D60 sim.)
- Output Transform - CSC - ACEScct
With this setup all I needed was a simple node right at the front to do some slight ‘corrections’ to make the ACES version of the LUT more closely match the ‘original’. It looks quite close in my eyes. Have a look 
