Note that the Exution Role should have been arn:aws:iam::**:user/** not arn:aws:iam:::user/ as shown below due to code formatting.
Anyhoo, after returning to this and attempting to to define a new API action integrating with a AWS service from scratch I got the following message:
AWS_IAM Auth type is required for AWS integrations with identity forwarding specified
Naturally searching for this statement generated absolutely nothing so in a rare moment of inspiration occasioned by a cup of coffee I tried:
arn:aws:iam::<my aws account>:user/*
And lo and behold the waters parted and the save button worked!
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