Hi Ian
Have just done the same for my M102.982 engined W124 (as part of renewing the cylinder-head gasket job), it's pretty easy.
The tensioner cover bolt needs a 32mm socket:
- undo the tension from the serpentine belt & remove it from the alternator pulley
- remove the alternator upper bolts and swing it down anticlockwise
- undo the tensioner cover bolt and catch the spring as it pops out
- clean the block or oil & dirt around the tensioner's bore
- assemble the new tensioner (push the piston in until the first "stop")
- insert the tenstioner in the block, and insert the spring - tighten the tensioner body to 10NM
- ensure the sealing ring is in place
- screw on the tensioner cover - this will auto-tension the device
- tighten the tensioner cover bolt to 70 NM
- refit alternator, the ribbed belt, and tension it
I did all of the above on mine with the engine fan & cowling removed, it was pretty easy
