The mitochondrial DNA could not have come from Roman troops. Troops are male.
You get mitochondrial DNA from females, only.
All males and all females get mitochondrial DNA from their mothers, but no males pass it on.
On the other hand, no females carry Y Chromosome DNA.
The reason archeologists study mitochondrial DNA is that it's much more resistant to decay than nuclear DNA.
Mitochondrial DNA is inherited through the mother's line. We all have mitochondria and we all have mitochondrial DNA that traces back through maternal bloodlines. A roman soldier could have had a mother or other female antecedant who was Romani.
"The mitochondrial DNA could not have come from Roman troops."
I was not suggesting that a find from the 11th century had anything to do with the Roman period. What I was calling attention to was the fact that populations from Eurasia were at various times introduced into the British Isles, over and above the Ango Saxon conquests. There may be all kinds of surprising genetic traces found in the future in many places as the number of studies increases.