The ABS fault wont stop the car starting. It actually comes up for many many reasons, if anything at all may possibly have an effect on the systems working, it shuts down and gives the warning. Most likely whatever is stopping the car starting is the real issue and the ABS is a side effect that will go away on its own. What do you mean by "It tries to start"? What actually happens? Have you checked the battery? This so far really sounds like a flat battery issue. Or possibly an earthing problem. Not being a mechanic or electrician, I'd try the following steps before forking over any money.
Test the battery voltage - if you don't have a multimeter, go into engineering mode by holding down the left steering wheel button and turn on to ON (it' one of those buttons on the left side, the scroll wheel I think).
Check the terminals are on properly and clean.
Check the battery earth strap.
Check the bolt that holds the positive jumper terminal in the engine bay. (I hear it can cause problems if it isn't tightened properly).
IF it is the battery and it's new, make sure to check the alternator too. Simple indication is the charging voltage, it should be somewhere in the range of 14 - 15.5V for a healthy system (VEs vary the voltage, so don't worry if it moves a bit).
As for the left rear ABS, yes a lot of people seem to have a problem with the loom for it. But that's if there is an actual fault with it. Also that isn't always the problem, I had a sensor die but have never had an issue with the harness. Either way, it won't cause starting issues. Besides, you really should check what the issue is before just replacing parts because some people have had a problem with it. Things get very expensive guessing just replacing stuff without any checks to see what's most likely.
Good luck, hope it's not too serious (read expensive).
