OK, you pointed me in the right direction: The March 2019 edition of
The Trow (which you can find here:
https://cotswoldcanals.com/the-trow-archive/) explains that the intent is to build a new, lower level channel alongside the River Frome where it passes under the M5.
I
think going into the Frome itself was considered too risky by the Environment Agency for reasons of flooding and ecosystem, so the "separate but parallel" solution was arrived at.
The Highways England grant is superb news. The third obstacle, after the A38/A419 and the M5, is of course the Bristol-Gloucester railway: but I believe the intention is to construct a culvert there at the same time as a long-planned engineering closure, coming up in a couple of years.
I'm hoping Phase 1B will be complete before we move our mooring from Worcester - as another long-term CCT member (and the original creator of their map) I can't wait to take our boat to Stroud for the first time.
(A pedant writes: this bit is of course the Stroudwater Navigation rather than the Thames & Severn Canal.)