Sue, I'm currently writer-in-residence at a London uni and have found the following really useful when working with postgrad/undergrads- particularly the first two:
Grammar, Style & Usage
D. J. Collinson, Writing English: A Working Guide to the Skills of Written English (Wildwood House, 1987)
Biber, Conrad, Douglas, Finegan, Johansson and Leech, Longman Grammar of Spoken and Written English (Longman, 1999)
Gordon Jarvie, Bloomsbury Grammar Guide: Grammar Guide (Bloomsbury Reference, 2000)
Randolph Quirk & Sidney Greenbaum, A University Grammar of English (Longman, 1973)
Rebecca Stott & Peter Chapman, eds, Grammar and Writing (Longman Speak-Write, 2001)
Rebecca Stott & Simon Avery, eds, Writing with Style (Longman Speak-Write, 2001)
William Strunk Jr, & E. B. Whyte, The Elements of Style (Allyn and Bacon, 1999)
Michael Swan, Practical English Usage (OUP, 1995)
Joseph M. Williams, Style: Toward Clarity and Grace (OUP, 1995)
Some useful online grammar resources:
Purdue University’s Online Writing Lab
http://owl.english.purdue.edu/
Like many of the best US sites this is aimed at both students and tutors. It’s impossible to list all the things you’ll find at Purdue but online resources include: printer-friendly handouts on writing concerns; PowerPoint presentations on writing skills, grammar, spelling and punctuation; and links to other online writing resources.
Diana Hacker
http://www.dianahacker.com/
Diana Hacker was one of the most successful writers of writing textbooks in the US. Nearly half of all colleges and universities there use at least one book from her range which includes A Writer’s Reference, A Pocket Style Manual and Rules for Writers.
Diana Hacker’s site has useful links to Student sites and Instructor sites as well as lots of intriguing features like ‘Diana Hacker’s Language Debates’ which is a series of mini-essays on style and usage issues.
The Everyday Writer
http://www.bedfordstmartins.com/Lunsford/meveryhm.html
Like Diana Hacker’s site and Purdue, The Everyday Writer site also has links to web resources and online writing exercises. And it also has really useful areas like ‘The 20 most common errors’ and how to fix them.