Kat I've heard of this, it's common over here, plus more macabre things placed there too...

My grandma though - had a Louis something ormeleu (spelling) gilt clock bricked up in to her fireplace in the house they had in Royal Tunbridge Wells... She hated it, (this would have been 1950's)... And when they were having work done to their fireplace, she persuaded the workmen to have it bricked up in the fireplace - so she'd never have to see it again. (She was positively a woman to herself a real once in a lifetime sort of gal)...

Fast forward to the 1990's, we were calmly watching Antiques Roadshow after Sunday tea at my parents. It was showing an Ormeleu clock, and they were raving about it (it was ugly). Gran calmly announced, oh, I used to have one of those... hated it... had it bricked up in the house in Kent... We were stunned... it was worth an awful lot of money!

from a website found... There's a lot of customs and superstitions about what objects are placed where - google them, it comes up with interesting info...

Renovations revealed 'a George III coin, dated 1765, embedded under the hearthstone of the original fireplace. It is well known that the builders placed a penny in the hearth when building a house and other finds relating to witchcraft and superstition have, from time to time, been found behind hearths or lodged on a stone in a chimney or in the roof space of cottages' - Elizabeth Gale, Farmers, Fishermen and Flax Spinners (Privately, 1983) p58