I don't think a DA42 diesel v. TB20 comparison is very useful because the diesel has a huge advantage in fuel availability in Europe and even more so in the 3rd world.
But, FWIW, I have flown in the original DA42 TDi and it was doing 140kt at a combined fuel flow of ~ 11 USG/hr which is almost exactly what my TB20 does. This makes sense since the two have similar volume cockpits, the DA42 has the extra drag of the 2nd engine, but is more streamlined, has a higher engine compression ratio, and there is no free lunch
At FL100, the TB20 does 140kt TAS on about 9.5 USG/hr, at peak EGT, 2200rpm.
The UK diesel v avgas price comparison is difficult too because they have just stuck a tax on diesel for "leisure" use, which you are supposed to self declare. However "training" is exempted, AIUI, so I guess everybody is doing a lot of training flights now. I really have no idea how this is supposed to work, and I don't know avtur pricing around Europe. Certainly the avtur tax in private use kills the diesel engine market for private-aircraft retrofits, totally.