Fod Focus 1.6Tdci 2011

Hi Guy i am new to this forum, but i have been reading the articles on the forums as a non registered user. My first question is is it true that diesel give less mpg in the winter and more in the summer. The reason i am asking is because my focus would do about 52+ mpg around town. I am not a boy racer and drive very carefully. Just recently it has dropped to below 46mpg.

Second Question. Does the quality of the fuel have an impact on the mpg. I normally fill up from BP, my mileage is not brillant 525 miles on a full tank. This is what the on board computer say. But when i fill up from Shell it say either 595 -625 per tank full.

Thirdly How do you know if the dpf is playing up. I normally do 60 miles per day. But i stick to about 55-60 miles per hour just to get the mpg.

Sorry about the long winded questions. I have had the car since october 2014. I love the car alot better than my previous Vauxhall Astra. Many thanks Guy.