Well i set out to find the reason why this issue seems to occur so infrequently, causing so much grief when it does occur. After spending the afternoon on this I think i found a consistent pattern and a simple fix around the issue.
After trying many things like creating a different workspace, installing the standalone Force.com IDE, and simply restarting the computer, all to no avail, i found that there were some projects in my eclipse ide where content assist (CA) was working correctly. The projects where CA was working were mature projects with many different types of components whithin them.
The new project which i just created today only had the new trigger i just started writing. In this project CA was not working. So after a few hours i made the connection that the mature projects were working correctly as a result of the components within them.
Here is a temporary solution:
I added a class to the project which only had the trigger and WHA LA!! content assist started working for both the new class and the trigger.
I will be reporting this bug to the Salesforce guys.
Hope this helps others.