I agree that you'd need the output of a cowshed to produce enough pee (assuming Hazlenut knows what he's talking about and as far as I know the jury's out on that :-)  I would also doubt that the average olive root goes down 15 metres.  More like one metre.  Olives are evergreens and I've never seen them shed their leaves.  
Anyway Colin, you say the trees are close to the house.  Have you done any building work lately.  I was wondering if cement etc. could have washed into the soil.  Do you prune and water them at all ?
Like I said, I'm not an expert.  Probably best to get a local guy to look at them.