Hi folks,new to the group so thanks for letting me join! I'm trying to replace the A/C compressor but don't know how to ease the tension on the fan belt. Any ideas or hacks would be appreciated,this ain`t the weather to be without A/C! Thanks in advance. Simon A.

There is quite possibly an auto-tensioner which is acting against the belt to keep correct tension.
Usually requires a spanner of the correct size to turn the tensioner away from the belt

Unfortunately this didn`t solve the problem,there are 2 tension rollers,top and bottom.You have to rotate the whole unit anti clockwise with a spanner until the 4.5 mm hole at the bottom is lined up with one behind it,insert a pin to immobilise the unit,this loosens the serpentine belt so you can slide it off the pulleys.