I was surprised but delighted to find this large colony of fungi on a pile of woodchips earlier this week. I think these are Cellar cups (Peziza cerea), as they fit perfectly with the description in my guide book:

Pale cup associated with limestone and often found growing out of mortar joints in buildings. Solitary or in small groups on woody debris, sandbags, limestone rubble, lime mortar and, sometimes, in damp cellars. Fruit bodies to 5cm across irregular cup-shaped, with the margin inrolled at first; inner surface pale ochre or yellowish brown with a smooth texture; outer surface concolorous and finely granular.

However, there are over 50 species of similar cups in Britain, many of which grow in similar locations, so microscopic examination is needed to be sure of identification.