Posted: 18/03/2014 at 18:02
Hello again - sounds as though you've had to do what was needed here about 20 years ago, ref a better soakaway. The last time I had the system emptied & cleaned - a couple of months ago now - the chap said "it's a good system, this one! They don't make them like this nowadays".
This for Artjak - The lids of the two adjoining tanks in the system here are at ground level, and the brick chambers are approx 4' x 4' x 4'. What happens is that the waste goes through the drains into the first tank, the heavy/semi-solid stuff settles in the bottom part of the first chamber and when the level reaches a certain height the (mostly) water at the top drains into the second chamber. This settles too, and eventually the (by then much cleaner/clearer) liquid passes into the run-off pipe when the level in the second tank reaches a certain height. In other words, the tanks don't simply fill up, thus needing to be emptied. It can - and does - sometimes take years, depending on the construction and sizes.
It sounds to me as though there may well be some sort of blockage in the OP's neighbour's system - tree roots are sometimes the cause. The fact that the OP mentions a willow tree raises the issue - willows are notorious for having a large and vigorous root-spread.