Chosen Solution
Wen I replaced my hard drive there was one retaining bracket holding the drive in (see hard drive replacement step 11 - MacBook Pro 15" Core 2 Duo Models A1226 and A1260 Hard Drive Replacement) and one was just lodged under the hard drive - so I left it out when re-assembling. Was it serving a purpose or just left there by an apple mechanic by accident?
There is only one bracket. Somebody must have left it there.