Snow in Summer or Cerastium Silver Carpet. It's in the Carnation family. A low growing evergreen ground cover. It can be a thug in the flower border.

Sandwort (Arenaria Montana) perhaps?

Or even on of the dwarf Achillea. A. x huteri or A. ageratifolia.

I have an Arabis that looks like this but tiny mauve stripes on the flowers.

I have just had a second thought and I believe its Helianthemum ( Rock Rose) I'm sure I've seen it somewhere.

I think you were right the first time Jimmytheone - its definitely not one of the Helianthemums, but may well be Arenaria, one of the sandworts. I thought it might be Arabis initially, but the flowers on this have five petals, not four, so its not that either. Trouble is, there are so many flowers, its next to impossible to see the leaves clearly, but Arenaria gets my vote.

A close up of a flower and a leaf would help no end, but Arenaria seems right to me too.

Thank you all, very interesting.

I found the label.....Arenaria is correct....Avalanche