I’ve worked out the reason for my problem and a partial solution
The problem is a combination of two factors
(i) The pin graphic module-pin-topbg.gif runs across the top of the module – the top of the graphic is mid grey #222222 which matches most of the background, hence it is not visible when the module is not at the top.
(ii) The top 50px of the main background (up to the menu bar) is made from container-bg.gif and graduates from #222222 to #363636 to give a subtle change in grey. The result is that the top of the pin graphic stands out against the background.
I edited module-pin-topbg.gif to make the top transparent but it didn’t work – anybody know why?
My partial fix has been to edit container-bg.gif so that it is all #222222 which is an improvement but does lose the subtle change in background.