Hi ECH,
As a Deeply Discounted Security (DDS), a US Treasury Bond would normally be subject to UK income tax on maturity.
However, if redeemed or sold before maturity, the transaction would be subject to capital gains tax (CGT).
Any gain (or loss) should be entered in the 'shares and securities' section of the Self Assesment tax return (and any Foreign Tax Credit Relief claimed in the relevant section).
CG54602 - Deep discount securities: CGT adjustment
Thank you.