Why, had he been at it? Hope you gave him a good telling off.
Writing, sign, or visible representation...then racially aggravated...in a public place...those three elements are the gist of the of the most commonly used legislation for such matters. Its complex, but the way the case law has evolved around the meaning of public place also includes someone's private house where the source of the speech has originated elsewhere. The legislation applies to the street, the pub, and, as Jeff points out, the interweb.
Unfortunately, cyclists aren't a protected category in the pantheon of hate crime. However, there are plans afoot to include anyone targeted because of their appearance, mainly intended to stop gingerism and that sort of thing, but it probably won't be long before someone convinces a court that someone target because of wearing Lycra and cleats is included.