Stu, i'll try and get one up.
I don't think i'm missing anything as it was taken off the car just because one shocker was leaking.
I can't see for the life of me whats wrong though, although i'll be the first to admit it must be something as it certainly doesn't sound right.
Ive basically got the shock. Then the spring sat on the platform. Then the top spring cup. Then the top mount sat in between the spring clips of the shock. Fed this up through the rear shock tower, then put the top "cup" on, and put the nut on the top of the shock, tightened it all up and its like this.
The problem lies with the top "cup" having a 18mm or so boss sticking out of the bottom of it which bolts up tight against the top mount creating the gap.
It would seem someone before me has had the drivers rear shock off and encountered the same problem at some point, as they have ground the boss back to around 12mm or so. I got 2 new cups for the rear, and these have a boss of around 12mm ish from new, althought the gap's still apparent a bit