Jaks

Hi guys my e class 320cdi 2008 with 101k miles was averaging around mid 30s mpg around town but recently it's gone down to around 20mpg.There are no engine management lights coming on. One other unusual thing I've noticed is that when the engine is cold it does around 30mpg and just starts dropping as the engine warms up after which it shows 20mpg once warmed up.
been reading about maf sensor and o2 sensors but would have thought it would show as a fault on the instrument cluster.havent put it on the computer yet but was wondering if anyone has come across this one.
many thanks
jaks