Dark oil is good thing as it means it is holding all the impurities from the engine so they will be removed when drained. It's what it is designed to do
Unfortunately it could be the either the timing or oil pump drive chain about to let go.
Remove the belt again and then start the engine and let it idle for 30 seconds. If the noise is still there then it's the chain.
They are replaceable but quite and involved job as the engine has to be...
From the MOT manual:
Existing halogen headlamp units on vehicles first used on or after 1 April 1986 must not be converted to be used with high intensity discharge (HID) or light emitting diode (LED) bulbs. If such a conversion has been done, you must fail the headlamp for light source and lamp...
Never heard of them being called 'abrasive additives' before.
SAPS stands for Sulphated Ash, Phosphorous and Sulphur and the spec just limits the amounts of these that can be included in the oil as they can poison exhaust treatment devices when burnt.
Low SAPS oils just use different chemical...