I have to hand in a CV with all the jobs and dates I have done however I cant remember everywhere I have worked along with the durations...is there anyway to get this information online?
no....you have to contact your former employer for dates....or just guess! i only ever put month/year for previous employment and exact dates for current.
If you have had lots of jobs, and short stays at some of them, this may look bad on your CV.
I would only put a small number of the jobs down, the ones you stayed at the longest.
I dont like lying on CVs but you may wish to slightly stretch some of the start and end dates so the jobs flow from one to the other, rather than having lots of "gaps" in between.
Have you actually been asked to list EVERY job you have ever had? It's been a few years since I was involved in CV's, but most employers only seemed interested in the last six or seven years. Is this the first CV you have had to submit? What about P60 and P45 tax forms. Have you not kept your copies?
How long a period? Would bank statements have some info on tracking your wages going in with names of the payer? It might at least jog your memory. How about enlisting friends or family to help, a collective effort might yield more results.
If you can't find the precise information I think it's very unlikely indeed that a potential new employer would know enough about you to be able to dispute what you've put. I wouldn't worry too much