Thank-you all for your replies. Thanks for the suggestion of Autolusso, I visit that area so will try them in future though now I'm reluctant to go to another garage, having paid so much I am expecting the current garage to put it right, and I am sure they will once they (or we) work out what is wrong.
Yes, it has had new air/fuel/pollen/oil etc filters when serviced about 9 months ago. Yes, the engine is getting up to it's normal running temperature.
I haven't measured how much diesel, it's just that relative to previous consumption it's using now, at a guess, four times as much, about a sixteenth of a tank to do 25 miles. And it's lost that Alfa 'let's go' feeling.
Cheers to you all for your thoughts and suggestions