Thanks for all the ideas.
I have contacted Hennessy who suggested to look first at the seams. I have strung up in the backyard with some light rain and under daylight it is clear that there is a constant weeping through the seams coming in. So this is certainly a significant part of the problem and maybe the entire problem (though the water was coming in from all parts of tarp on the night - maybe this was simply the sheer volume of it running down the sides from the apex and going everywhere).
Suspension is definately not part of the problem - drip lines are working and suspension just inside the drip point remains dry.
I'll see what Hennessy come back with from here and keep you all posted as to the answer.
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