My Fabia 1.4 TDI is the first diesel I have owned and I have noticed recently that whereas the price differential between diesel and petrol was 4p during the summer, it is now about 8p. At the same time, my normal mpg has dropped by 3-5mpg. Could both of these facts be a result of the seasonal introduction of "winter diesel" ?

Definite winter diesel , 8p difference but will soon be much more expensive fuel is about to go up big time :( I've got the same as you 1.4tdi and the 1.6crtdi which I feed with vpower which is way more expensive

Story I've heard is that in winter , diesel competes with heating oil ( for central heating) , so the price increases .Suggest you subscribe to www.petrolprices.com ( free registration) an look for the places where it's lowest . Don't forget that at 45 -50 mpg , a few miles out of yer way can see a decent profit ,as well as making a point to those who overcharge . I've got a local place ( few hunred yards up road ) charging over the odds ,with another a mile away at least 5p lower . I got ot the lower price one - on two scores

1) more that do this passes back the message to the overchargers

2) those lowering price get the benefits of more custom

Heating oil & derv is same stuff or quite near to it chemically I think ( not sure if chemically is the right word ) but you'll understand I think ??

Yes you are mostly right Seb. They actually add a small amount of heating oil to some diesel, plus what they term flow improvers other than heating oil. Not sure what the flow improvers are as such. Just keeps everything flowing nicely in freezing conditions and stops your fuel filters blocking with wax.