Removed and hair-dried heatsink-cooling fan, and used brush and many tissues to clean old stuck dust.
But because I'd never seen thermal paste and mine was grey and scattered, I thought it was stubborn dust, so I accidentally wiped it all off from heatsink and CPU.
Connected everything back up and 15mins later casually launched speedfan.
Alarmed by the 90c temps, I restarted and saw 84c at BIOS.
I googled the colour of thermal paste and realised what had happened.
That dry grey 'scattered dust' I wiped off was actually paste and somehow kept my PC working at high, yet workable temps all these years.
This must be a good custom PC as I never replaced thermal paste since buying it in 2008.
I got Amazon to rush-order same-day delivery (thank God as no shop stocks it locally in London, and ones that do cannot be reached as the trains are on strike, and taxis are expensive especially since it's Black Friday).
I bought Arctic mx-4 due to quick-dry and 8 year longevity once applied.
Isopropyl was impossible to buy (even amazon can't deliver until after weekend), so I used tissue and microfibre to clean old paste crumbs before.
I diagonalled 2 lines of the paste on CPU, placed the Heatsink-fan on top, took it out to see that paste spread most of the space, with only a small gap at one side.
Cleaned a little excess with tissue.
At BIOS 10 mins after stabilizing:
CPU= 40c, MB= 39c (this is only 1 Celsius less than before I removed the heatsink-fan).
10 mins after booting to windows,
Idle and Firefox (after loading):
CPU= 45c, Core0= 45c
CPU & Core0=63-68c