You can buy "* NEW * Original Fastener - Intel socket T 775 heatsink" on ebay for 75p.
They come in 2 pieces...this is how you fit.
Fitting the new fastener
First, insert the white plastic motherboard anchor into the heatsink bracket - from under the heatsink so that it fits as shown in the photo (A). Then orient the black plastic post to rest on top of the anchor as shown in the extra photo (B). Note that there is a small "key" moulded into the top of the anchor - this fits into a slot in the post. Check orientation with the circled area in the extra folder.
DO NOT PRESS THE TWO REPLACEMENT PARTS TOGETHER YET!
Double check!
Check again!
Only when you are happy that everything fits correctly and is oriented correctly (check with the other, good, fasteners on your heatsink) it's OK to press the black plastic post gently but firmly (only approx 4mm) onto the white motherboard anchor.
There! You've sucessfully replaced your broken heatsink fastener. Now you are ready to fit the heatsink / fan onto your motherboard, following Intel's recommended procedure.
Removal...
obviously, remove the broken/damaged fastener before you start!!! And make sure that you do not bend or damage the metal heatsink bracket whilst removing the old fastener. (The white moterboard post will literally fall out of the heatsink bracket if you get the angles right.)