The WWW is so vast, how do I discriminate between good and bad sites? I can easily spend an hour searching and not find anything particularly useful.

Try http://www.intute.ac.uk/ - Intute is a free online service funded by JISC providing you with access to the very best Web resources for education and research. The service is created and hosted by a network of UK universities and partners. Subject specialists select and evaluate the websites in the database and write high quality descriptions of the resources.
I recommend that you click on the “Resources for FE” section of the site which is organised in familiar subject areas. Each sub-section includes a link to a subject specific search facility, a link to a subject specific tutorial and most have links to case studies which provide ideas on how to use electronic resources for learning & teaching.