Thank you for taking the time to reply to my question. As it has been a little while since I first posted it without response, I did some further research and it seemed that the Intel heatsinks would not be compatible thanks to the 'unique' manner of mounting CPU coolers to the Dell motherboard. So I had no option but to source a used Dell heatsink from Ebay. It wasn't cosmetically perfect - who is going to see it anyway? emoticon.Wink.title - but it seems to be working well with no problems during a very long Windows 7 install and update.
So to add my own little bit of advice to the discussion for anyone considering the easier-to-get Intel heatsinks, don't bother, because they won't fit!