I'm not a huge fan of Ditra matting. My distrust of this product though stems back 20 years when it was completely and entirely misunderstood.
Shops were selling it as some form of super replacements for all sorts of solid substrate suggesting you could use this stuff to tile straight on floorboards, floating floors the lot. I had to correct so many failures.
So rather than become misinformed I spent a long time on the phone (at the time) with Schluter technical. Fortunately now much of this information is available in easy downloads - here's a great info sheet:-
http://www.schluter.com/media/DITRAHandbook-ENG-2013.pdf?v=201306080603
If you look through you will see without exception that the tile is fitted over the Ditra with unmodified thin set mortar. That means no polymer additives at all to comply with the correct installation guides of Schluter.
Things have developed a lot since I stopped fixing but certain constants remains, if I understand this correctly, once you used a polymer modified adhesive on top of Ditra matting the uncoupling mechanism ceases to work or is reduced in efficacy substantially. In so much as it's no longer truly operating as a decoupling membrane.
How does this then dovetail with the BS standards requiring polymer modified adhesives to be used in these situations particularly with UFH?