I have been looking at a few websites which help to identify problems, here is a selection.
Here is one from the BBC which asks you a series of questions
An illustrated identification chart from theseedsite (scroll down for pests around plant)
Help and advice from the Royal Horticultural Society
Garden friends and foes - lots of pictures of creatures you share the garden with.
GardenOrganic also provides lots of information, even more for members.
If you have any more information or suggestions, please leave a comment.