Can anybody confirm if these are Achillea Ptarmica please ?

The flowers certainly look like they could be a double form of Achillea ptarmica, but there several other similar flowers (such as double forms of Petrorhagia saxifraga or Gypsophila paniculata). The foliage doesn't look typical, so I'm not sure. Sorry. Maybe Leftwood can help.

Enlarged photographs show leaf serration. I'd say it's Achillia ptarmica.

Is this any better ?

Never coming across this concept before, it seems that they pay for there extravagance by being sterile.

So I'm not likely to see many of them down the same street.

Clicking on the third photo in your initial post shows leaf serration, a characteristic of A. ptarmica which, though perhaps not widely grown in your area, isn't sterile.

I would suggest it is Achillea ptarmica "The Pearl"..which is the common one grown in UK with double flowers.