Hi @Olly_Warren,
From reading your post is sounds like you’re doing things correctly.
From my own experience, I am able to request a listing of all webhooks for given workspace using Asana’s NodeJS client library by using client.webhook.getAll(<workspaceId>)
If you’re using NodeJS/MongoDB, May I suggest you take a look at Asana Webhooks Manager on Github, it’s a starting point application I recently put on github, to assist use cases likes yours, and save others valuable time
Eyal