On Sun, 10 Feb 2019 16:47:38 GMT, Harry Bloomfield
wrote:

snip

I dusted the entire PCB, case and air ducts out thoughroughly with a
paint brush,

A note on that, it's recommended to use a natural bristle brush or a
conductive (carbon?) one to reduce the risk of generating static
electricity firing the process. I use a 1" quality badger hair brush
and try to hold the metal ferrule whilst using it (so that I act as a
conductive path to earth for any static).

snip

My best guess as to the problem - maybe a bit of loose solder or
conductive dust shorter something out. The brushing cleared it out.

The classic issues are a blocked vent at the exhaust (I've extracted a
4mm thick layer of what looks like felt many-a-time), the fan fins
themselves or a blocked input vent grille (less common as many have
multiple air input paths). That and / or the thermal paste drying out
(and so needing re pasting).

I have also recovered the odd mounting screw that has fallen out and
then lodged itself somewhere subtle but they generally rattle about as
you move the laptop around (and sometimes also tapping it to dislodge
it again). I have found loose solder balls but they don't generally
seem to cause any issues.

Well done for giving it a go and welcome to the broken clip thing. ;-)

Cheers, T i m