I guess I should have been more specific. There was no need to remove the heat sink to clean the fan, you could have done that with a brush or compressed air can. You only remove the heat sink to change the thermal paste. If you removed the heat sink you MUST remove the old paste using 99% isopropanol or ethanol and apply new paste. If the plastic fasteners are broken, you should probably get a new CPU fan. They are rather difficult to use, unfortunately, and I had a hard time removing and replacing it. I did write some notes on it tho:

Even so, it's difficult to get it right. Maybe other fans have more sane attachment methods.

Ha! I think the damage is done now, but you are right it was a lot trickier than it ought to have been getting the blasted thing out and in again. I did watch a youtube vid (his fan was a screw in) of a guy removing his cpu fan and heatsink before my attempt, but he made no mention of the need for re-pasting, or indeed bothered to. It seems most likely that i've broken something on the motherboard with my 'hammering', which is more than a little annoying, the whole idea of having to replace the thing! Someone has pointed me to a site that sells replacement pins, so guess i'll try eliminating this possibility first off. And I suppose I ought to source myself some isopropanol or ethanol, as well as some thermal paste too...

Your guide will come in handy in my next attempt at least, so thank you.

Just to say thanks to everybody who helped me here, especially H_TeXMeX_H. I got a new cpu fan and heatsink installed at my local computer shop today, and thank heavens there was no damage to the motherboard! This fan is an improvement on the last, and I am now getting lower readings on cpu temp + speed. Still some noise out the comp, but not like before, and I think a healthy amount that I could not reasonably ask for less.