Time Machine stopped backing up iMac

My Time Machine has stopped backing up my iMac. It is a 2Tb TM with 693GB still available and has been working fine since I got it out of the box in August 2012.

Watching the process a few times it seemed to get stuck on calculating changes. Investigating this I then followed instructions to delete com.apple.TimeMachine.plist from the root library, restart the Time Machine and iMac, then reconnect the Time Machine to the iMac, at which point there is a 120 second countdown and the backup starts.

No luck. Still calculating changes. I let the process just run for days and days without altering anything on my iMac. No backups taken.

I followed instructions to verify disk and it said that there was descrepancy. So I then followed instructions to boot on MAC OS X installer disk a verify and repair hard disk. Result was that after repair it said everything looks OK.

Again retried the reboot and iMac/Time Machine again several times, gets through scanning and then preparing items but never actually backs up.

Downloaded the widget and it says that UUIDs don't match and goes into an indexing routine;

Waiting for Index to be ready (101)

for quite some some the says

Node requires deep traversal:/ reason:must scan subdirs|new event db|

I leave the process to run but never seems to actually backup. I get an error

Error: Flushing index to disk returned an error: 0

I then leave it to run on its own schedule for several days but still not backup complete.

Time Machine says last backup was 28th May 2014.

I should add that the same Time Machine is regularly backing up my Macbook Air as normal without the same issues. My iMac is OSX 10.6.8.

How can I get Time Machine to start backing up my iMac again?