If the gills are white then the mushroom is possibly poisonous. The cap and gills of the The Destroying Angel, the Spring Amanita, the Clitocybe Dealbata and the Clitocybe Rivulosa (right) are all white, and all are deadly. So a white mushroom with white gills is generally something to avoid.
My guess Paul is it is one of the above but is in it's advance state going off white and upturned. My very limited knowledge of mycology is 'never eat a mushroom with white gills'.