Protect data while laptop is at repair shop I've got to give my computer (mid 2012 MBP, 15", running OSX 10.10) to a repair shop (not Apple store, but Apple authorised) for them to replace the battery.

My concern is that someone at the repair shop could either access data on my drive or install something malicious. 

I've got FileVault enabled and a secure password to log in to the machine, is there any further steps I could take (short of removing the HD prior to giving it to them)?
If you have FileVault enabled there is no need to do anything else to secure the boot volume of your hard drive.

So if there is only one volume on the drive you are fine. Just shutdown the Mac from the Apple menu before handing it in to be repaired. They will not be able to open the drive (or boot the Mac of it).