One thing that might be worth looking at is at the next service, get the garage to check the state of the particulate filter (or check it yourself if you're feeling handy). Cheap supermarket diesel may seem like a good cost-saving move in the short-term, but the amount of crud that ends up in the filter from using it consistently is truly shocking. The tip I was given was to fill up every 3rd tank with one of the detergent-heavy fuels (e.g. V-Power diesel in the UK) and take it for a high-speed motorway burn at the same time once a month or so. Of course, if a previous owner made a habit of only using the cheap stuff even that won't clear the crud, and the filter may need to be serviced or replaced.
Keeping the filter relatively clear will improve your fuel economy considerably no matter the season.