Hi Cat, sorry for the silence. The business has gone a bit crazy, the garden is in full production (I grow veg and soft fruit) and time seems to just run away with me.

I haven't contacted Dartington yet, but showed my brother and sister in law the photos of my bowls. They are great Dartington fans, and they used to go the factory shop at least once a year. (I'm going back about 30 or 35 years here). They both say the bowls are typical of what would turn up in the factory shop as trial pieces. If something had been tried but wasn't considered suitable for mass production, any saleable pieces would be in the shop. And that's what I had thought right at the beginning, when I couldn't find them in the Smithson book.
I do keep checking any other likely suspects though. For example, today I've gone through all the Walther / Mikasa pages on Replacements.com, because they use that textured surface in quite a few pieces, and they're quite adventurous with the design of their rims.
So my bowls are currently on the "naughty shelf", not forgotten, but still not identified.

Meanwhile, I captured one of the most amazing pieces of cobalt blue cut to clear glass about 6 weeks ago. I'll be asking for help with it later today.
Thank you for all your help so far. And no, I'm not on the South coast (I'm slap bang in the middle of England), but I bought mine from a car boot on the south side of London.