Unable to use the Alexa REST API to update the "datastore" for an Alexa Widget

I have written a simple Alexa skill which works and I've now added a Widget, these both display as expected.

I am now trying to use the datastore but every time I try to update the store, I am getting an error related to not being able to find the Client ID from the skill metadata.

I am using the ClientID & Client Secret from the Permissions section of the skill in the Alexa Developer Console, using this and the alexa::datastore scope, I am able to authorize and get an access_token.

However, when I the try to call the Data Store REST API to update the datastore, it fails with the following error:

{
  "type": "INVALID_ACCESS_TOKEN",   
  "message": "Unable to find the Client ID from skill metadata:amzn1.application-oa2-client.xxxxxxxxx"
}

I have tried running the Plant Care Widget example skill as it uses the datastore, but that also returns the same error if I look in the CloudWatch Logs.

I have spent a couple of days trying to get this working to no avail.

I am sure my ClientID & Client Secret's are correct, as the authorization works, so it feels like its either some other configuration issue, or a platform issue, or maybe I missed a step that is required?

I also wrote a quick Powershell script to test the API which you can see below:

$clientId = "your-client-id"
$clientSecret = "your-secret"

$body = @{
    grant_type     = "client_credentials"
    client_id      = $clientId
    client_secret  = $clientSecret
    scope          = "alexa::datastore"
}

$tokenResponse = Invoke-RestMethod `
    -Uri "https://api.amazon.com/auth/o2/token" `
    -Method Post `
    -ContentType "application/x-www-form-urlencoded" `
    -Body $body

$tokenResponse | Format-List *

$accessToken = $tokenResponse.access_token
$accessToken 

$headers = @{
    Authorization = "Bearer $accessToken"
    "Content-Type" = "application/json"
    "Charset" = "utf-8"
}

$timestamp = (Get-Date).ToUniversalTime().ToString("o")

$payloadObject = @{
    commands = @(
        @{
            type      = "WRITE"
            namespace = "debug"
            key       = "ping"
            data      = @{ ts = $timestamp }
        }
    )
    target = @{
        type  = "USERS"
        users = @(
            @{ userId = "dummy-user" }
        )
    }
}

$payloadJson = $payloadObject | ConvertTo-Json -Depth 10

$response = Invoke-WebRequest `
    -Uri "https://api.amazonalexa.com/v1/datastore/commands" `
    -Method Post `
    -Headers $headers `
    -Body $payloadJson

$response.StatusCode
$response.Content

Any help you can give will be much appreciated!