[BigAl33]

typical hey brit!!
i asked the same question a while back
Alfa Pages :: View topic - Water pump life expectancy

 

[BigAl33]

The tensioner pulley is pressed onto the arm, the nut to the right of the tensioner locks the tensioner in place and if undone the whole thing will come off.
You should be able to press off the tensioner pulley off the arm and press on the new one.

DO NOT HIT THEM OFF WITH A HAMMER AND PUNCH!!
The arms are cast iron and will crack.

 

[BigAl33]

can be done with a bolt, different sockets and a vice.
I did my 16v this way, someone who has done an 8v will have to advise you

 

[BigAl33]

With the 16v there are 2 types of tensioners, different hole dimensions, check if the same applies to the 8v, would be very annoying for the replacement ones not to fit