I checked with Dartington, via their website, about the dates the embossed logo was used (this was a new logo for me). A very prompt and kind response was given by the Head of Design at Dartington Crsytal (Torrington) Ltd. Part of the response is given below:
The backstamp you describe was used to mark products made for Dartington's Studio Collection, the pieces were made at Dartington's Barbican Glassworks which was situated on the Hoe in Plymouth. The studio has now closed, we just have a retail outlet on the site, it was in production from 1999 until 2004.