run 1.0 t+e from perm L and N to the DP isolator
then run 1.0 t+e to a one way single pole pull switch
then run 1.0 3 core+e to the timed fan
connections
DP switch L+N in, L+N out
pull switch..... both browns in common (L)
black sleeved with brown in L1 (sw L)
blue and grey sleeved with blue in a separate connector block (N)
fan..... brown goes to L
black sleeved with brown goes to sw L (or T)
grey sleeved with blue goes to N
i think this is the way to do it. completely independent of the light. make sure you get the feed from the perm live and neutral of the lighting circuit.
and the DP wall isolator outside over the door will effectively isolate all 3 poles of the fan.