I know I will get alot of flames on this one but my advice is find a shop
that does this all the time and has a good reputation (there are tons of them
in the US but I don't know about the UK).  Get them to do it.  15 mins later
you are driving away slightly poorer, but you have no stress.  If they break
the windscreen, they bought it.  If you break it, you bought it.  Also if it
leaks later you have someone to bring it to, if you do it, you are stuck
trying to fix it.

If this was something majorly expensive I could see doing it yourself, but I
bet an average shop here would charge you $50 US at the most for replacing
the seal.  It's worth it to me.

Just my opinion.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242