Get user from accountId

I’m trying to get a user by their account id
https://api.atlassian.com/ex/confluence/xxx/wiki/rest/api/user?accountId=123456%3A5792ad64-1234-1234-1234-abcd76b19e4a

Using an oAuth bearer token - but I’m getting a 401. The URL is encoded to replace the : with a %3A
I have read scope, and can pull other data OK.

thanks
Paul