viscous fan fell off

Hi All
I noticed today that the viscous fan on my defender 200tdi had fell off and was wedged inside the cowling[no damage done].
I replaced the fan back onto its spindle but could find no way to tighten it down[ as the spindle and fan rotate together under pressure] .,or secure it in place.
So my question is how do you lock the fan in place,? And any ideas as to why it has come off in the first place.

My question is

You need to use a tool to hold the pulley the fan mounts on still then do it up tight

I take it the 200tdi is the same as the 300tdi and a LH thread.
It really should be self tightening. Just screw it on and tap it with a fan spanner, it should not be coming off as long as everything is OK.

Cheers

O.K Thanks .
sounds like I need to buy fan spanner. .