As far as I am aware it goes something like...

Henge
+ heel stone(s)?
+ station stones

and then onto the fiddly bits with the sarsens, including the distraction of the mysterious concentric double blue stone circle with posible horizontal stones connecting each pair of vertical bluestones radially (never completed).

I have the English Heritage £12.50 guide and it tells you all about the rectangle (in full colour mind you), so I'll have a read through it this weekend and report back on monday.