Mary Harnes summer coat - any ideas about this label or dates?

Lovely - looks like spring personified! I am tending towards 70s - early. Unfortunately I don't know anything about this brand either.

 

Another vote for 70s

 

I did look up the history of the fabric Crimplene and of the company ICI which was apparently the largest British chemical manufacturer that made everything and anything kind of like DuPont.
You may already know this but here is what I learned:
Crimplene was launched in 1959 and enjoyed popularity throughout the 60s especially for A-line dresses but began to fall out of fashion in the early 70s due to the invention of lighter weight polyester fabrics like Trevira.

“ICI developed Crimplene, a thick polyester yarn used to make a fabric of the same name. The resulting cloth is heavy and wrinkle-resistant, and retains its shape well. The California-based fashion designer Edith Flagg was the first to import this fabric from Britain to the United States. During the first two years, ICI gave Flagg a large advertising budget to popularise the fabric across America.”
So based on that, it’s possible the dress is a bit earlier. For some reason though both the color and floral pattern strike me as very 70s.

 

Yes, ICI is a familiar name over here, at least to me. As is Crimplene. Crimplene was most definitely still around in the 70s. It fell out of favour gradually, and certainly wasn't over until well into the 70s. I expect it was still in use (though definitely not fashionable!) in the early 80s - think of older lady 'smart' dresses in larger sizes.

Personally I'm fond of a bit of crimplene!

I think this is probably very early 70s. It could just possibly be late 60s.

The care labelling is very minimal, just the one, which also indicates an early date in the time period, ie very late 60s to early 70s. Later into the 70s there would be more likely to be several care symbols, and in the mid-late 60s, probably none at all. This would vary by manufacturer, so isn't definitive, but that's my observation of care symbols in Britain. Some manufacturers started using them early and before they were mandatory, and some no doubt dragged behind.