Sudden mpg drop

My 58 plate Freelander 2 auto has always done around 33mpg. This is mainly on short journeys with a longer weekly run.
Over last couple of months this has started to fall - its now 26. Same use, same fuel. Feels a bit sluggish. Has been serviced - after fuel issue noted and new fuel filter fitted, brakes etc all checked.#
Never had a warning light come on.
Went into LR dealer as a last resort. Test driven, diagnostics etc. Result "there is nothing wrong with the fuel consumption and nothing wrong with car".
Anyone else had a drop off in economy like this? To say that I'm unimpressed is an understatement.

TD4 I assume? Leaking injector?

My wife's fl2 dropped by 10 mpg, it didn't matter how steady you drove it, on the instant display it was 25mpg at 60mph in top gear. on the way home from work I drove the car as normal until the engine was at normal temp , i then hammered the arse off it for 10 miles. The following day I went to work and back in car and sure enough the instant display at 60 mph in top year was 40 mpg. I suspect DPF partially blocked but no warning light on

I had an 08 GS FL2 a few years ago, on which I put 90,000 miles in 3 years so experienced quite a few services. I always found a loss in mpg after every service and it only picked up again after a couple of weeks of motorway driving.

Ramheads solution may well be worth a try though.

I'd be inclined to clean the MAF sensor. Seems to cure a lot of ills and apparently easy to do (although I never have).

Winter diesel doesn't help.