First thing you can do is, if you have an Android smartphone or tablet, get a Bluetooth OBD2 dongle from eBay and the Torque app from the market. Check for error codes. It's worth the investment and may save you spending out on parts you may not need. 
Poor MPG, my first port of call would be the lambda sensors - but there may be fault codes if this is the case.
Poor MPG, my first port of call would be the lambda sensors - but there may be fault codes if this is the case.