Are most kayaks now made from plastic rather than GRP? Does anyone have
manufacturer links, shops etc?

Google is a wonderful thing. Then again, try
http://www.s106156335.websitehome.co..._retailers.htm

Most playboats / creekers / river runners / general purpose boats are
plastic. Proper sea-kayaks are GRP. Opens can be some form of plastic -
usually.

I commend http://www.ukseakayakguidebook.co.uk/ and
http://www.ukriversguidebook.co.uk/forum/ to you as other sources of info.

Enjoy - Mike