if it is brown, it is likely (but not definate) that it is old sludge - so yu might be lucky. first step is to remove tank and clean it with washing up liquid and a bottle brush to clean it. Flush it well before putting it back! That will give you a known clean starting position from which you can flush the system etc. if it comes back - yu know the sludge is being made afresh. check the dipstick to see if the oil is contaminated. Are you loosing water?
evening Adz - i think yu owe us a pint in the Gaylander