one of the pulleys has a bolt, I think it is the second one down, this when turned clockwise releases enough tension to allign belt. Position of belt around pulleys is not obvious but can be worked...
Its just a gearbox code, F20 is usually paired with the C20XE engine found in Astra GSi, 16v Calibra's, Cavailers in the 80's and 90's. Smaller engines cars had the F16 and the Calibra and Cav Turbo...