thanks for the advice guys but had the hdd out and that was actually very clean, was expecting that to be caked in dust. but as iv said above i was only watching netflix for less than 10 mins from power on and it crapped out. so very doubtfull its heat build up especially as id been on it for 6 hours the day before with no issues.
had something similar when i did get the YLOD a couple of years ago, one day fine and then beeeeep.
as for your heat sink chapparal, you really need to clean it more often, mines done every 3-4 months for reference btw.