use wp_get_attachment_url in a foreach loop

I have about 200 posts each with 1 image attached.

At the moment each post has a custom field with the key as ‘image’ and the value as the attachment ID, i need the value to actually be the image url.

I have this sorted for all new posts, but I need to run a one off function to get the attachment url for each existing post and save it as a custom field.

I have tried running through all posts with a foreach, grabbing the attachment id, and running wp_get_attachment_url() but i cant get it to work.

Any help appreciated