It IS possible to replace the tensioner and belt WITHOUT removing the Viscous fan! On a 1.8T 20v AEB. I found the hex bolt holding the fan impossible to crack, even with correct size locking pin in place!!
After slippng the aircon compressor belt off(awkward) the old serpentine belt can come off. There is very little room for manouvre but the belt can be slipped through the gap between the blades and over the short belt pump below the alternator. It is not immediately visible but after many angles of observation and swarf in eye, broken fingernails etc etc, i managed to get the belt in position.
The tensioner can then be bolted on and then you need and assitant!!one open ended spanner on the lug to release the load(a short pipe may help the leverage) and then some one with small thin hands(missus or ET) can feed the belt into correct place before releasing the spanner and putting the load back on the new belt.

I cranked the engine a couple of times without letting it start to ensure correct seating. Job done. Saves a few quid from the factors and only basic tools required, plus a lot of PATIENCE!!