I have a Volvo V50 1.6 Eco Drive. Over the past few months, it has started to lose fuel efficiency dramatically. It dropped below 52 MPG and ended up around 42 MPG. It turned out that this is caused by a semi blocked diesel particulate filter. This was probably blocked because of soot caused by having dirty injectors. I bought a bottle of DPF Cleaner and also some Injector Cleaner and added them to the fuel. I did it over two half full tanks. I found that the MPG increased dramatically over the period of about 2 weeks.
The other potential diagnosis for this problem is a dirty MAS (Mass Airflow Sensor). You can get this cleaned also.
If you are in the UK then I recommend visiting Halfords and getting them to clean your diesel particulate filter in a liquid.