I've found it!! And you'll never guess where!! Well, I'll put you out of your misery ..............................................
BHS!!
I haven't been in there for years and there were actually 3 dresses that were strong contenders. I'll attempt a link:
www.bhs.co.uk/mall/productpage.cfm/bhsstore/877792/230325
The dress I bought isn't the same fabric - mine is a striking floral black and white - but the style is identical. I am absolutely over the moon, as you can probably tell!!