Well, actually, it was also twenty one a few days ago when we took this photo. You may have seen similar markings appearing on the pavement in various places in the town centre. We don’t know why they’re there, but we guess that they’re something to do with marking the boundary of property that the borough council has bought recently. (You’ll see that, in the middle of the ‘zero’, a metal stud has been inserted in the pavement.)
(As an aside, we recall – rightly or wrongly – that cafes and pubs that have permission for outside seating in the public thoroughfare are supposed to mark the seating boundary in some way. There are studs outside The Bear – which might just be for this purpose – but we can’t, offhand, think of any others.)
CORRECTION. We’ve now seen a similar stud and marking in Knoll Walk. As far as we know, this doesn’t have any link with property recently bought by the borough council, so it looks like we’re completely wrong about the purpose of the studs.