Thanks for the tip; I did in fact open her up and replaced the thermal grease (admittedly this needed doing), and cleaned out the vents a couple of weeks ago. However, I've checked sensors now that I have lm-sensors installed and I'm seeing uncomfortable core temperatures: near 70C. I'm going to have another look. If I end up needing to invest in an external fan to place the laptop on I will. Thanks again for the advice.
[update] And we're done. One thing I'd obviously forgotten to do was blow out the actual heatsink; I'd only cleared the grill on the laptop cover. Little globules of dust came out through the fan. I'm praying that this will have helped. I've checked the sensors now; temperature hits 65 at high load (burn test launching three youtube videos at once with my usual suite of apps open), after ten minutes, then stays there. That's still a bit warm for my liking, but the fan never went to its higher speed, so on that basis we'll see. Thanks again for the advice - I'll be a bit peeved if I find out all of this was caused by some dirt in the heatsink but very grateful nonetheless.
[edit:] On an unrelated note, I've just noticed something about that css that I was working on that, had I not lost the original and started again, I would've missed. Found out I can avoid absolutely positioning most of my divs, to be precise. Perhaps this was a blessing in disguise (happy again
[update:] While running cool, without the fan changing speeds, I'm now getting (sigh) hard freezes. Am opening a new a thread, marking this one "solved"