SP-API: 'Access to requested resource is denied'

Hello,

I'm having some trouble getting my first SP-API request to work. I have followed all the instructions on the Selling Partner API Developer Guide (https://github.com/amzn/selling-partner-api-docs/blob/main/guides/en-US/developer-guide/SellingPartnerApiDeveloperGuide.md).

My requests appear to authenticate correctly so I believe the signature etc is valid. However I'm receiving the following response:

{
errors: [
{
message: 'Access to requested resource is denied.',
code: 'Unauthorized',
details: ''
}
]
}

I believe this is something to do with the IAM configuration. I have a user with a "SellingPartnerAPIPolicy" permissions policy attached. I also have a "SellingPartnerAPIRole" Role defined to which the "SellingPartnerAPIPolicy" permissions policy is also attached. I am using the user credentials in my request.

Can anybody point me in the right direction to resolve this?

Many thanks,
Meriadoc.