While it's been cold out everyone's mpg has dropped a bit, but I appear to have lost nearly 20 of them!
While I don't think even freezing weather can account for that much, and my tyre pressures are fine, my attention turns to the oil.
I do a large chunk of motorway miles and I'm about 500 miles away from the next service so could the oil and filtersbe like sludge and blocked? Do you guys reckon it'd be worth my while changing the oil halfway through the interval or should it really be fine, or is this what I should be expecting!

I can't imagine how the oil could be totaly responsible. Even if it had transformed into stuff more suitable for a back axle.
Very cold weather will markedly increase starting injection priming, initial running richness till the engine gets up to 20C and then you should be on to your usual warm up figures. Running perhaps a fraction richer because the intake air is colder and denser. Once warmed up it should not be significant and there might be a couple of extra horse power to play with.
What car, do they have a reputation of running cool in cold weather, is your temperature gauge showing the engine running cool? Might be a case for checking thermostat is working properly and the engine is getting properly up to running temp. Apart from that check the figures again.
I was going to ask 'Whatever happened to radiator muffs' (kitchen foil radiator blanking for the cheapskates) but then remembered they invented thermostats that work properly.

LOST 20!
Blimey what did it do to start with!
Do you have a lockable petrol cap!!

Bit more info needed........

I have also had my MPG reduce, by quite a lot,
when i bought my Mondey TDCi it was always in the 46mpg bracket, and that has now reduced to 35
Most of my driving is local, at this time of year, and longer runs in the summer towing a caravan about

so a couple of weeks ago, i suspected the DPF getting blocked, so drove up the M23 at 4k revs for 20 5 miles, and just by doing that mpy MPG has risen to 38/39 so i do suspect that in the colder air, the cars are not getting up to temp, and using more fuel than normal, a bit like having the choke out for longer

Steve

It's a 206 2 litre hdi, over summer I was getting just shy of 60 now more 42-45. It's running a little colder in general, but just enough to notice on the gague (something like 85 in summer and 80 now) but I figure that's bexause I've got the heating on.
I guess the dpf could be a problem, it only gets to 2000rpm when I'm up to speed and I rarely do above 60. Maybe I should see how quickly I can get to work tomorrow morning

I noticed this on Friday night with the SAAB, usually get around 40mpg with the 9-5 Aero, on a motorway run. Only got
30mpg on a 80mile round trip, nearly all on the M1
This is petrol, by the way.

Blocked exhaust will kill economy, give it an Italian tune up!

Change Air filter as well as oil.
Air filters are cheap enough.
My MPG dropped and did not run as smmoth for a while, turned out to be the EGR valve.
I cleaned that, and it was better for a couple fo days.
Changed the EGR valve in the end costing £240 all in all. (did it myself)

Checked brakes not binding.... Had that on my Land Rover, one piston sticking.

The last 2 diesels I've played with (Volvo d5 and jag x type tdci) both had their egr valves massively clogged. The Volvo worse so with the intake being gunked up aswell. in some places the tubing was over 75% blocked. On both cars the egr valve, tubing and intake manifold were thoroughly cleaned out and performance was better and the economy jumped a minimum of 10% on a combined cycle. Very diy'able job too.

Can I take it the egr is a screw out and scrub job?

depends on the engine, as its a common thing there are usually guides on the internet. The jag was 2 bolts 2 pipes and a plug. You will need some rubber gloves and some sort of degreaser as the gunk inside is nasty oily crap. If I was running a diesel for any length of time I would see if a blanking mod could be done without freaking out the ecu.

Have you checked your tyre pressures? I wouldn't think it would make that much difference but it could be part of your problem, and always worth doing as it's usually free at the supermarket.

I'd look at the air filter - all my diesels in the past have been better on economy after a service and the only thing I could put it down to is the air filter.

Tyre pressures are fine, I've been rather anal about that as the last set leaked.

After all this searching there is not one single guide! Not even on the 206 forum. Bit of a bummer as I don't even know what I'm looking for. Oh well to halfrauds on wednesday

Little update, I drove most of the way to work in 3rd and 4th yesterday morning (she handles rather differently when being driven like that!)
On the way home there was a noticeable difference

Looks like giving the dfp a good blasting again is in order, thanks guys!