Fuel usage

I have a 2014 diesel octavia estate 1.6 blk edition. Lately, and since service my mpg is alot lower than normal. I understand my car is old but recently ive noticed im using more fuel.

Ive asked my usual mechanic and hes replied with the expected answer i thought hed suggest which is weather, tyres, speed, fuel prices and that mpg is never ro be trusted. Im not taking fuel prices into consideration as im seeing my fuel being used up more. Mpg today has been 34,on a route i normally use and get 55mpg.

I use the car everyday to commute to work using same route and speed and definitely see variation in mpg amd fuel consumption.

Any tips or ideas?