Hey there, RodMaughan
Welcome to the Microsoft Community forums.
According to the FAQ here:
https://help.minecraft.net/hc/en-us/articles/360041345271-Minecraft-Dungeons-FAQ
WHAT IS THE DIFFERENCE BETWEEN THE ‘WINDOWS 7 AND ABOVE’ VERSION AND ‘WINDOWS 10’ VERSION OF THE GAME?
The ‘Windows 7 and Above’ version of Minecraft Dungeons is downloaded through a launcher program that is compatible on those operating systems.
The ‘Windows 10’ version of the game is delivered directly through the Microsoft Store application and does not include a launcher.
Gameplay is not different between the two versions, only the method of accessing, downloading, and updating the game.
NOTE: Purchasing the game on one digital store does not grant access to the game on the other store (i.e. Purchasing on minecraftdungeons.net does not make you own the game on Windows 10 Store, and vice versa).
There is no difference between the gameplay. The only difference is that one requires the launcher, and the others don't as they are Windows Store Apps.
In terms of getting a refund, what I would recommend, while you're waiting for a support agent at Mojang, is checking their refund policy here:
https://help.minecraft.net/hc/articles/360030463592
As you're purchase is within 15 days, you should be good to go. However, you do need to contact them to initiate a refund.
Hope this helps,
Regards,