I've cut away the lip circling the hex nut so I can now get that bit firmly in the vice but the next bit is my vice isn't fixed to a table so I'll need to drill some holes and get some long bolts...
The large nut looking piece is part of the axle the tensioner pivots on.
It bolts into the engine front with the boring as a seal.
The nut shape needs holding securely and the outer face with the female Allen key hole needs to undone. Then you...