Sorry, I wasn't being clear. It's fixed! I'm not sure how it happened, but somehow the assignments managed to get different end dates to the task so the "Task View" looked like this: (hope whitespace works properly)

-Task1 35 days Thu 11/04/13 Thu 30/05/13
- Bob Bobson 0 days Thu 11/04/13 Thu 11/04/13

So Bob Bobson was down for that task, but his assignment was 40% of 0 days. Changing the assignments end dates to match the task end dates fixed the issue.

The only thing is how it got like that in the first place, I'd never been in the Task view before.