I am resident in the US, paying tax in the US, on an L1 visa. My employment contract remains with the UK company.

When I return to UK at the end of my contract (not sure if it will be 2 or 3 years yet), I won't be earning any income in the US and have no property here. Will I still need to continue to file US tax returns as well as UK ones?

You would have to file a tax return only for the years you are actually present in the US. So once you return to the UK, you would file your final return for the year you left the US. Depending on your residency status for that year, you would file as a non-resident alien or resident alien (or dual status alien if you apply for an early residency termination date).
You should read publication 519 which has all the relevant information regarding tax returns for aliens.
http://www.irs.gov/pub/irs-pdf/p519.pdf

You can also order a copy by mail on the IRS website:
Forms and Publications by U.S. Mail (http://www.irs.gov/uac/Forms-and-Publications-by-U.S.-Mail)

Great, thank you. I hoped that would be the answer. I had been told different, but from a source wanting to be helpful, rather than one particularly informed. Thanks for the link.

TaxForAliens has again provided all that is needed.