It’s my understanding that diesel is sold as different mixes depending upon the season. My assumption is that winter diesel has stuff in it that stops it from going funny at colder temperatures – vague recollections of lorries sat at the sides of snow covered roads with small bonfires under the diesel tanks.
So, assuming that I might have about 1/3rd of tank left of summer diesel (last filled up in early June), how long should I leave it before going for winter mix? Have suppliers changed to the winter type yet? Will it cause any problems if I leave it as long as possible before filling up again?

Diesel waxes [ thickens basically] at low temperatures – i forget what exactly but its below freezing so no rush

Many cars pre heat the fuel to avoid this anyway.

I am not sure if we do get “winterised” diesel as you suggest.

before it goes well below zero…

Not really worth worrying about

Don’t think this happens in Uk, but will ask at work, as the front part of where I work is a fuel station.

We do get winterised diesel, which is one reason your mpg drops in the winter a bit. I guess you’d be ok until several degrees below.

depends on the circumstances, but the basic principles of your thinking are correct in my experience (2 years as a front line oil rep in the late 1980’s selling fuel/lubes to farmers/hauliers etc, 12 years in the industry overall). It depends where/how you bought that old fuel, originally? I’m certain forecourt fuel will have the appropriate additives all year round, certainly in 2013. The only occasion where fuel would possibly not have the appropriate winter additives would be commercial bulk purchases by farmers and hauliers into their own bulk storage tanks. More likely Farmers who would buy Gas Oil (tractor diesel), which is diesel without Duty added. Even in the old days I’m pretty sure I only sold winter additives for Gas Oil (tractor Diesel) not regular Diesel, but it was a long time ago.
In short, if you filled up at a forecourt pump, then almost certain no winter issues with the fuel.

Many cars pre heat the fuel to avoid this anyway.

Not in this country i’m affraid , they are simply not required .

Mountainous regions near the Alps will get a more winterised version, so those off skiing will generally tank up at the bottom of the hill. I know at least one who had wax issue parked up in Tignes last December, cos he’d tanked up before the Alps.

But what’s sold at a forecourt in the UK will be more than adequate year round in the UK, unless perhaps there’s some freak -20C weather.

Banana it could be an issue if you don’t drive for a long time. Like me now. Car’s full of diesel from last month, not using it.