Water pumps are so cheap you may as well change when doing a timing belt,
a seized pump or tension roller bearing will snap a belt ,so worth checking and replacing when needed
don't know where you got the 100k belt interval from
think it used to be 7 years or 70k but was reduced to 4 years 40k years ago (depends on what engine i guess)
some engines have a thermostat behind the belt too (which can be re-located to the top hose ,to aid replacement of the stat in future helps mpg and keeps breathers clearer)
there are lots of different belts and kits ,so be sure to check what you order with what engine you have fitted ,because ebay fitment info is not always correct .
many jobs on an astra are simple compared to omega (wheel bearings for example)
one thing I noticed recently is your not far from the sea side
try and avoid buying puff the magic dragon cars
(that have lived by the sea)
salt water feeds tin worms
i'm sure you know that ,but may be helpful for anyone who is reading this who does not