I went back again today for another look after confirmation that my coordinates were good.
I wasn't holding out much hope as I'd found bits of ziploc bag there last night. But something that the cache owner had said made me want to go out and recheck GZ.
So off I traipsed with the small dog and went and had a better look at GZ ...and I found it, exactly where I looked yesterday. It was massive. How did I miss it?
So now I can finish off my series knowing where I can put my caches.