Getting specific image thumbnail urls of attached images

I am using the Manual Image Crop plugin to set thumbnails for images. I am not using any images as ‘featured images’ however.

I have a number of thumbnails saved to each image. If I use wp_get_attachment_link(id, 'medium'); then I get the desired ‘medium’ thumbnail returned but as whole html block output, including ” tags, size etc etc.

I just want the URL of this specific thumbnail (or the img tags ‘src’ attribute). Is there a built in wordpress function for this or will I have to use PHP regex or something?

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