Borrowings at market value is the company's estimate of the "fair value" of its borrowings. The current estimated fair value of the company's borrowings is based on the redemption yield of the relevant existing reference gilt plus a margin derived from the spread of BBB UK corporate bond yields (15 years+) over UK gilt yields (15 years+). The reference gilt for the secured bonds is the 6% UK Treasury Stock 2028 and the reference gilt for the perpetual debenture stocks is the longest dated UK Treasury stock listed in the Financial Times.