UKPassats

Hi I have a 2.0 b3 Passat owned for 4/5 weeks I used to get 40 mpg easy from this car but has started running lumpy and getting really rubbish mpg it gets 30mpg which I was not worried about but then once I've been out in it turn it off and then back on I'm lucky to do 16mpg it's driving me mad... And costing me a fortune in fuel
Car has new spark plugs leads cap rotor arm...
Timing has been set all marks up perfectly then had the digi tune up guide set up done for 6btdc

Any ideas what could be wrong