I've just checked Plant Finder on the RHS website, but they don't currently list any suppliers.
You could try contacting some specialist rhododendron suppliers to see if they have one tucked away somewhere - try Glendoick, Burncoose or Loder's

Thanks- I've tried all three and no joy- as I live in Cornwall perhaps I should take a cheeky cutting- any recommendations??!

As someone who opens their garden for charity, I would be very upset if someone did this in my garden. I suggest a polite request for a cutting would be more readily received from the owner