I don't know hee hay about 1.2 engines, but most new cars get fitted with an automatic auxiliary belt tensioner which needs to be turned back with a spanner to slacken the tension so that you can replace the belt. As you seem to have discovered, movable alternators seem to have disappeared!
Clue, if the above is correct WRT the 1.2 engine, then my advice is to jack up the O/S front and remove the wheel and wheel arch liner to get better access to the tensioner as it will want to move quite a long way after you have taken the belt off - so your ring spanner might try to disappear out of sight!