Mk2 Cmax 1.8 tdci random reving to 1800 ish revs

[ed1808]

hi there many thanks for accepting me to the forum.
My father has a 2008 mk2 1.8 tdci duratec and it has started ramdomly reving up to 1800 ish revs on idle and sometimes when comming off the throttle when approaching say traffic lights. i also personaly think it has a slight tappet sounding knock also. the 2 issues are propably not connected but i guess u never no. 
if im right in thinking a diesel will over rev if it over fuels then im hopeing somebody can help with this. 
i have a basic fault code reader along with forscan and it only gave me codes for various light bulbs.

any help appreciated regards ED

[ed1808]

no ideas on this one chaps n lasses? please