I found the time where I had to force restart my computer, and have pasted the 10 minutes previous (there's lots of things that say 'failed':
26/04/2013 21:23:48.563 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
26/04/2013 21:24:58.623 com.apple.quicklook.satellite[1289]: bootstrap_look_up2 failed with 0x44c
26/04/2013 21:24:59.000 kernel[0]: Sandbox: sandboxd(1290) deny mach-lookup com.apple.coresymbolicationd
26/04/2013 21:25:06.114 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
26/04/2013 21:25:07.021 sandboxd[1290]: ([1289]) QuickLookSatelli(1289) deny mach-lookup com.apple.PowerManagement.control
26/04/2013 21:26:15.649 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
26/04/2013 21:26:28.713 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
26/04/2013 21:26:30.777 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
26/04/2013 21:26:49.120 com.apple.usbmuxd[44]: _heartbeat_failed heartbeat detected detach for device 0x26-192.168.1.64:0!
26/04/2013 21:28:36.000 kernel[0]: (default pager): [KERNEL]: ps_allocate_cluster - send HI_WAT_ALERT
26/04/2013 21:28:36.000 kernel[0]: macx_swapon SUCCESS
26/04/2013 21:29:24.890 mDNSResponder[56]: 72: Could not write data to clientPID[-1]() because of error - aborting connection
26/04/2013 21:29:24.894 mDNSResponder[56]: 72: DNSServiceGetAddrInfo v4v6 a1505.dspl.akamai.net. PID[-1]()
26/04/2013 21:29:24.895 mDNSResponder[56]: 73: Could not write data to clientPID[-1]() because of error - aborting connection
26/04/2013 21:29:24.898 mDNSResponder[56]: 73: DNSServiceGetAddrInfo v4v6 a1505.dspl.akamai.net. PID[-1]()
26/04/2013 21:29:24.898 mDNSResponder[56]: 74: Could not write data to clientPID[-1]() because of error - aborting connection
26/04/2013 21:29:24.903 mDNSResponder[56]: 74: DNSServiceGetAddrInfo v4v6 a1505.dspl.akamai.net. PID[-1]()
26/04/2013 21:30:20.293 com.apple.usbmuxd[44]: _heartbeat_failed heartbeat detected detach for device 0x28-192.168.1.90:0!
26/04/2013 21:30:21.000 kernel[0]: Sandbox: sandboxd(1310) deny mach-lookup com.apple.coresymbolicationd
26/04/2013 21:30:22.759 sandboxd[1310]: ([191]) WebProcess(191) deny file-read-data /Users/Luke/Library/Preferences/com.apple.ServicesMenu.Services.plist
26/04/2013 21:30:22.773 sandboxd[1310]: ([191]) WebProcess(191) deny mach-lookup com.apple.pbs.fetch_services
26/04/2013 21:30:35.457 AddressBookSourceSync[1312]: *** -[IADomainCache init]: IA domains cache couldn't be read.
26/04/2013 21:30:42.121 com.apple.backupd[1318]: Starting automatic backup
26/04/2013 21:30:42.187 com.apple.backupd[1318]: Backing up to: /Volumes/BACK-UP/Backups.backupdb
26/04/2013 21:30:45.709 com.apple.backupd[1318]: Using file event preflight for Macintosh HD
26/04/2013 21:30:46.124 com.apple.backupd[1318]: Will copy (14.6 MB) from Macintosh HD
26/04/2013 21:30:46.133 com.apple.backupd[1318]: Found 487 files (27.7 MB) needing backup
26/04/2013 21:30:46.138 com.apple.backupd[1318]: 620.8 MB required (including padding), 955.88 GB available
26/04/2013 21:31:01.379 com.apple.backupd[1318]: Copied 1769 files (30.4 MB) from volume Macintosh HD.
26/04/2013 21:31:01.393 com.apple.backupd[1318]: Using file event preflight for Macintosh HD
26/04/2013 21:31:01.402 com.apple.backupd[1318]: Will copy (17 KB) from Macintosh HD
26/04/2013 21:31:01.403 com.apple.backupd[1318]: Found 26 files (17 KB) needing backup
26/04/2013 21:31:01.404 com.apple.backupd[1318]: 587.6 MB required (including padding), 955.85 GB available
26/04/2013 21:31:03.903 com.apple.backupd[1318]: Copied 428 files (17 KB) from volume Macintosh HD.
26/04/2013 21:31:03.968 com.apple.backupd[1318]: Created new backup: 2013-04-26-213103
26/04/2013 21:31:04.826 com.apple.backupd[1318]: Starting post-backup thinning
26/04/2013 21:31:06.225 com.apple.backupd[1318]: Deleted /Volumes/BACK-UP/Backups.backupdb/Luke’s iMac/2013-04-25-205656 (12.9 MB)
26/04/2013 21:31:06.227 com.apple.backupd[1318]: Post-back up thinning complete: 1 expired backups removed
26/04/2013 21:31:06.264 com.apple.backupd[1318]: Backup completed successfully.
26/04/2013 21:33:07.844 Dock[147]: no information back from LS about running process
26/04/2013 21:33:12.100 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
26/04/2013 21:33:40.643 WindowServer[95]: CGXRegisterWindowWithSystemStatusBar: window b already registered
This is where it froze, I kept the computer on to see whether the music would keep playing past the current song, which it didn't, so I turned it off. It then goes on to boot:
26/04/2013 21:40:15.000 bootlog[0]: BOOT_TIME 1367008815 0
...
I can paste the rest of startup as well if it helps?
I will repair disk permissions but it may be worth pointing out that I usually do this every month anyway - even in the months leading up to the hard-drive failure.
It might also be worth pointing out that I haven't ever had a backup drive plugged in at the time of a crash before.