Easiest and probably most accurate information is to read, and when required refer to, the Driver's Handbook (Owners Manual), if you do this you could know more about the car than some long term owners. If you haven't got the printed paper copy then you can download a pdf copy from the following link. It's not always in the best English but it will tell you about your car, Dr Google can often throw up errors and misunderstandings. - https://manual.skoda-auto.com/004/en-com/Models
Again read the Owner's Manual and you'll find exactly what it is and how to set it, it sounds to me as possibly someone else has set this this and you've not turned off or altered their setting. Driving using cruise control or automatic cruise control isn't always the most economical way to drive sometimes a driver might do better.
Remapping might be more fashionable than productive, timely servicing, maintenance and repairs are the best form of tuning for the whole car (and not just the engine) and really must be done before any further tuning will be fully effective. You can't fully progress unless the basic foundations of timely servicing, maintenance and repairs have properly fully been dealt with.
Another good tuning process for any vehicle is driver training, and it's mostly transferable to other vehicles. Most men have too much ego to think of further driving training.
As you know cars are dirty things but diesel is particularly dirty so you want to keep things as clean as reasonably possible for the car to run well and last well. As with computers GIGO (garbage in garbage out) with cars SISO so you want as clean as possible going in. This is from having a good air filter condition, perhaps occasionally using the more expensive Premium diesel fuels which offer additional lubrication and cleaning agents and to have timely changes of engine oil and filter.
My wife's previous car was a diesel, our one and only, the very few times I refuelled it I put in the more expensive cleaner fuel and when I floored it from low revs off a roundabout I'd see a big grey cloud in the rearview mirror instead of a big thick black cloud of **** with ordinary diesel fuel so I follow my experience rather than what others tell me. Even using this as the car done a lot of short journeys it still needed good long runs on motorway/dual/fast A-roads to give it clear out runs. Sustained higher speeds/revs than usual.
If you're worried about mpg do remember to give some latitude to the mpg dash readout figures especially as you're driving along. Confirm the car's figures for tank refill by how many miles since the previous fill (subject to both being to a full tank, first click of the station fuel pump) divided by how many litres it takes to refill the tank. If you want mpg divide the litres by 4.546 to get the number of gallons (4.546 litres to one UK gallon).
Good luck.