How on earth do you acheive the simple job of tensioning or replacing a fan belt

Hi and welcome to OOF.

The tension is taken up automatically on a V6 and probably on the other variants as well - no doubt someone else will confirm.

To replace the belt a ring spanner is placed on the tensioner and used to relieve the tension on the aux belt thus allowing removal/replacement.  Its a bit of a complex routing, however, so best to take a piccy first to ensure that its easily and correctly replaced.

When you get the chance, please update your profile to show location/car and engine so that others can help quicker and easier, perhaps being able to pop round if you are nearby.

Petrol engines: 15mm rinf spanner on spring loaded tensioner
25TD BMW engine: PITA, stupid bloody design for the spring loaded tensioner, and the tensioners are weak, and break.
2.2DTi: No idea....