Thanks for your suggestions. I emailed Andy Ford at Quiet PC technical support about this problem and got the following reply ...
"Sorry to hear about the problem.
I do get a similar enquiry like this about once a year and I always recommend using sticky thermal pads. I use a thermal pad on my own Southbridge and it's the next best thing to having it clipped onto the chip.
We don't sell them but I get one from Kustom PC's, who are a very reputable supplier - http://www.kustompcs.co.uk/acatalog/info_3769.html
Hope that helps!"
I have ordered the thermal pad that Andy suggested. I will give that a try, and if the bond does not seem strong enough, I will try the thermal adhesive that you have suggested. I don't have any problem with the bond being permanent, as I don't expect to have to remove the heatsink again for any reason.