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Results are presented from a series of comparisons between two independentfemtosecond frequency comb systems at NPL, which were carried out in order toassess their systematic uncertainty. Simultaneous measurements with the twosystems demonstrate agreement at the level of 5 x $10^{-18}$ when measuring anoptical frequency against a common microwave reference. When simultaneouslymeasuring the ratio of two optical frequencies, agreement at the 3 x $10^{-21}$level is observed. The results represent the highest reported level ofagreement to date between Ti:sapphire and Er-doped femtosecond combs. Thelimitations of the combs when operating in these two different manners arediscussed, including traceability to the SI second, which can be achieved withan uncertainty below 1 x $10^{-16}$. The technical details presented underpinrecent absolute frequency measurements of the $^{88}$Sr$^+$ and $^{171}$Yb$^+$optical clock transitions at NPL, as well as a frequency ratio measurementbetween the two optical clock transitions in $^{171}$Yb$^+$.

Evaluating the performance of the NPL femtosecond frequency combs: Agreement at the $10^{-21}$ level