"Step foot". With apologies to the American contingent here, stupid Americanism!! Because it has been creeping into British English these last several years, I believe, from American English.

Step doesn't work like that. You can step up, step out, step back, step on; you step in a direction. You can take a step, and a foot is also a measure of length. But you don't "step inch / metre / whatever". You can set foot; you are putting your foot somewhere (in it, possibly...). But you don't "step foot". Stupid, meaningless, idiotic misuse of the English language, whether it's the American or British version!!