I know the type you mean. When my Mum first moved to Devon she planted some mint. Unfortunately, she planted it on the shady side of a fence and it disappeared from our garden but went under the fence to next door`s garden where it flourished. When Mum wanted some mint, she waited until the woman next door went out and lifted me over the fence to pick it. She used to say "Its my mint anyway". It was dark green, slightly hairy with quite tough leaves. It also had a much stronger smell.