A stepped certificate's first-year rate tops the fixed ones: is its return higher?

Not necessarily. The rate shown for a stepped certificate in rate lists is usually its year-1 rate, after which the rate falls, while a fixed certificate pays the same rate every year. Comparing the two figures compares a number describing one year with a number describing the whole term.

Hypothetical example per 100 pounds over 3 years, simple return paid periodically: a stepped certificate at 24%, then 20%, then 16% gives a total of 60 pounds, an average of 20% a year, below its headline. Another stepped certificate at 24%, then 16%, then 8% has the same headline and a total of only 48. The headline alone cannot tell two very different schedules apart.

Even the average is not enough, because the timing differs: the stepped certificate pays more early and less late.

That is why a stepped certificate is described by its year-by-year schedule, and a fair comparison is on the money actually paid on your amount over the full term, for the same term and the same payout frequency. Ask the bank for each year's rate and the amount computed on your sum.