Natural or man made

Any thoughts on this artifact- a flatish quartzite pebble with a 10mm deep hollow chipped into one face? Found near Baginton, Warwickshire. A few Paleolithic tools have been found in the area.

Eek, I'm a year late with this response. looks very natural to me. It's an old fracture that has been rolled after its broken. 
However, it could now serve as a natural dish for earth pigments, a hot stone for mixing pitch pine glue, or maybe a lamp, fill with lard and light a reed wick? That's what I'd use it for now!