Hi Gordon, I can help here ...
First, according to PKF's Sierra Leone Tax Guide the withholding for residents is 30%, but for non- residents it should be 25%
Two possibilities there (a very restrictive definition of residency) or (Your UK company being conservative) , and of course they could be wrong, but their business is providing tax information for expatriates
I've posted the guide on my Google Drive for you to view: https://docs.google.com/file/d/1NRuJi-N2YkQEUm7n9e2GG9BFY2CjY6PVBb3sfaj3JLiZTXeO1g3YyYfazOYt/edit?usp=sharing
Now, in terms of your US tax, yes US persons ( and you would still be domiciled here as well as a resident, given your intent to return AND the term of the contract) all US citizens must pay tax on their worldwide income: Here's the IRS guidance: http://www.irs.gov/Businesses/Income-from-Abroad-is-Taxable
There is good news, however.
There is a tax credit on your US return for taxes paid to a foreign state:
There is a four point test
Four tests must be met for the tax to qualify for the credit:
Here's the IRS guidance on that: http://www.irs.gov/taxtopics/tc856.html
In terms of you rstate taxes, this is from Colorado:
ncome Tax - Living AbroadDoes a Colorado resident who moves out of the country have to continue filing aColorado income tax return?A Colorado resident reporting U.S. federal taxable income must continue to file Coloradoreturns as a full-year resident no matter how long he or she is out of the country. Mostsuch individuals are working on a temporary assignment, and return to Colorado.Because of this, there is a presumption that people from Colorado working in foreigncountries are still Colorado residents.All foreign income that is exempt for federal purposes is also exempt for Coloradopurposes.Individuals who abandon their Colorado domicile and become permanent residents of aforeign country no longer have to file Colorado returns. However, they would have to filea Colorado tax return as a nonresident if they had Colorado-source income (e.g., rentalincome). Such individuals bear the burden of proving their abandonment of Coloradoresidency. Continued Colorado residency will be presumed if the individual has notsevered all Colorado connections; for example, if the individual still carries a Coloradodriver’s license, votes in Colorado by absentee ballot, and/or still owns a home inColorado, or returns to Colorado.
In terms of reporting, Use line 21 on your 1040 to report income that is not reported on any other line.
In some cases, tax treaties with various countries will provide a method to waive the withholding (such as yours in Sierra Leone) but Sierra Leone does not waive withholding under double taxation treaties with ANY country, so you'll have to get relief there from the credit for taxes paid to a foreign country, mentioned above.
THis should give you everything you need
Thank you for the very prompt response. I would like to review your answers and check the links and may then have some questions.