Many thanks Niel for your very comprehensive answer.  Having written a paper about it, there is no doubt that you are GZ's resident 'Go To Guy' for any questions on batteries.

So then, taking your answer of 2000 cycles max for the LiFePO4 batteries, it seems that we would be looking at a battery life of 5.5 years, if they were charged every day by PV Panels, and then discharged at night.  Perhaps a smarter strategy would be to charge them one day, and let them discharge for the next 2 days, thereby getting double the life (in theory).  I'm not sure if any inverters out there are smart enough to do this, or if in fact the life would be increased, given that the depth of discharge would also increase. 
It seems like we would be getting something for nothing, and experience has taught me that such theories are seldom borne out in practice.

Say for argument's sake that the Tesla battery does in fact last 5.5 years, with an intial cost of USD3500, at exchange rate 0.75 works out to NZ$850 per year approx, or about $71 per month.  This then begs the question:  Is it worth spending $71 per month to save a proportion of our electricity bill, which might not in fact add up to that much?  I can see that some further calculations need to be done.