I am a foreign national who was in the US for 3 months in 2007 on a J1 visa. I was doing voluntary work and was not paid. It didn't occur to me that I would have to file a tax return as I didn't earn any money whilst I was there.

I have now been told that I did not have to file a tax return but that I should have completed form 8843. Is this something that I should do even at this late stage or should I forget the whole thing?

I don't intend on returning to the US except for holidays, but I'm a lawyer and would hate to be on the wrong side of the law!

Purpose of Form 8843 is to exclude the days in U.S. from Substantial Presence Test (residency test).

If you do not file Form 8843, and you were in the U.S. for only 3 months, you are still a nonresident as you do not meet SPT. You are not required to report your foreign income on the U.S. tax return.
So I feel it will not make any difference if you file or don't file Form 8843. http://www.irs.gov/pub/irs-pdf/f8843.pdf

Agreed. You did NOT meet SPT during your visit, so filing Form 8843 would be a waste of time.

Thanks a lot guys. You have put my mind at rest.

Glad to help!