They don't change to white - the native bluebell in this country is Hyacinthoides non-scripta, which is violet blue. There are other bluebells though, the Spanish version, and a hybrid between the two - the hybrid is Hyacinthoides x massartiana, and these come in blue, pink, white and variations in between, so that is most likely what your white bluebells are.

Am I rightv that the native bluebell has flowers only on one side of the stem whilst the hybrids and spanish ones can have them all round?

Yes Pamg you are right.

And every now and again you find a proper wild pink one(not a cross)

Thank-you everyone, I wondered if there was some kind of deficiancy in the soil.