Thanks for your reply Mark.
From my extensive Googling this afternoon I get the impression this is a problem with a number of causes so it’s hard for people to pin down exactly what might cause it for any one person.
As it happens, this just started working for me about half an hour ago. I’m not sure whether it’s something I did, but I *think* it was the following and it just took longer than I expected to take effect on the site.
Comment out this block in <b>/app/code/core/Mage/Core/Model/Session/Abstract/Varien.php</b>
// set session cookie params
It should start at around line 77.
This is obviously not ideal and it needs further looking into, but at the very least my client will actually be access his admin area over the weekend until I can look into it with fresh eyes on Monday
Hope this works for you, because I honestly feel your pain.