I have received the following email...
"I can confirm that planning colleagues are in discussion with Persimmon with the specific intention of ensuring that the outstanding landscaping/planting requirements are delivered as appropriate to the planning condition(s).
Head of Service : Sustainable Transport"
I had queried the planting planning conditions for the whole of the road, not just the roundabouts, and so, at last, Persimmon will hopefully fulfil their obligations as conditioned in the planning permission many years ago.