Stuart wrote:
I'm trying to figure out why the
window won't stay up as it won't have got heavier

There is a possibility that the window may have got heavier if the
glass has been replaced as this could be a different weight to the
original glass.

The other advice on things to look out for though is spot on.

This link may be of use.

http://www.sashwindowspecialist.com/diyguide.html

There is also a very good section in the Reader's Digest DIY Manual.