How to calculate foreign tax paid on foreign self - employment income

Help please.

I'm preparing a self assessment tax return for a UK resident non-dom.  He arrived in the UK from the US about 2 years ago.  He had UK employment income and US self employment income.  For the US income I have profit and tax deducted for the calendar year to 31/12/11 (I was just going to apportion this for 9 months).  My question would then relate to the 3 months income to 31/3/12 where the US tax liability has not yet been calculated or paid? Would I be able to offset any US tax?

Or do I ask for the results of the period ending in the year to 5/4/12 and see what tax has been deducted from those?

Many thanks