The picture shows the heatsink out of my laptop (enhanced with a few photo effects in the GIMP). It is bright and shiny except for the thick layer of dust coating the top of the copper vents – that would explain why it has been so noisy and prone to overheating in recent months!
I was getting frustrated by the problem and so ventured to open it up yesterday and undo a few more screws than last time I looked. As I eased the heatsink out and released the fan, the cause of the problem became obvious. A little bit of brushing and dusting (away from the upturned laptop, so as not to fill the cavity up with dust) and remembering to ground myself before reinserting the heatsink and things are running much more smoothly.
I’m amazed that the lack of cleaning hadn’t killed it. The machine is significantly quieter and the back corner where the heatsink is installed now feels cold to the touch rather than uncomfortably hot. Now I know what to look for, I should be able to avoid that particular problem happening again and any more pictures of the heatsink should be shiny all over.