I use Redex diesel fuel system cleaner £2.25 for 250ml bottle from Wilkinsons.
One bottle does two tanks.

pab wrote:

A cold engine uses more fuel than a hot one, and a journey which is short enough that 'I probably really should walk' suggests that the engine probably isn't getting up to temperature. You won't get a reliable mpg reading from a couple of weeks of that kind of use, so I wouldn't take too much notice of the 15mpg. On longer journeys on clear roads you should see somewhere around the low 30s for an auto.

It used to be that petrol was added to diesel in the winter to reduce waxing, and that could have a small effect on fuel consumption. I don't know if petrol is still used, of whether different additives are used these days. (NOT petrol these days - it damages high pressure injectors.)

you could always try 2TO in the tank - 300 mls to a tankful, repeat every refueling. There are those who say this is more effective than other petrol treatments for keeping major components clean and "lubricated".

Morrisons are selling it at £2 per 500 ml at the moment which is good - if you have a Morrisons near you.
I use it on the basis of it will do no harm. MOT in March may indicate if it has done any good when they test emissions!!