By, the wonders of the world wide web!

Ten minutes ago, I didn't know Cresset stones existed. Now I know that they are stone carved cups for use as oil and wick lamps, as used by medeival monks. I also now know they are not particularly common, with only 13 being recorded in England, and 1 in Wales, with the welsh one having the greatest number of cups.

However, they all seem to be rectilinear slabs, and fairly thin, so I'd say (on the basis of my new-found familiarity with the concept) that you're right there Paulus. Tiompan's find is not a cresset, no way.