What % of jackpot wins come from syndicates (as opposed to people who
buy tickets on their own)
This question refers to the UK Lottery

I have just contacted the UK National Lottery operator (Camelot) on
your behalf and they directed me to these results on their webpages.

Approximately 1 in 4 (or 25%) of the jackpot wins come from people who
win in a syndicate.  A ticket owned by 2 or more people is judged to
be a syndicate.

The largest win in UK lottery history was won by a synicate of 2
friends, in total they won £22,590,829.  There have been 3 instances
of a syndicate winning over £20 million.

These links (provided by the National Lottery operator) should help
you and back up the above facts:
http://www.national-lottery.co.uk/player/p/games/syndicateslanding.jsp
http://www.national-lottery.co.uk/player/p/games/syndicatesNews.jsp
http://www.national-lottery.co.uk/player/p/winners/facts.jsp#anchor03

If you have any further questions on UK National Lottery syndicates
please ask for clarification and I will try my best to help you out.

The UK National Lottery Help Desk (who are the best people to contact
regarding any queries) can be contacted here:
http://www.national-lottery.co.uk/player/p/about/contact.jsp

Good luck with playing the lottery!

Hilarious - they ran an advert for a while in early 2004 that shouted
"1 in 4 lottery winners are part of a syndicate".

This was meant to encourage you to join a syndicate.

They stopped running it - I guess someone realised it means that 3 in
4 lottery winners are non-syndicate players.  In other words ...
treble your chances of winning by NOT joining a syndicate!