In a word... Persimmon.
The green at the bottom of Sovereign Grange is still owned by them and the planting was specified in the planning permission with the, normal, clause that any plants or trees that die within 5 years from the completion of the development must be replaced.
However, given their complete disregard for other things mandated in the planning permission I wouldn't hold my breath waiting for them to honour their obligations and the District Council, the planning authority who should enforce these things, seem unwilling to take on Persimmon; presumably because Persimmon can outspend them one hundred times over should it come to legal action

It's sad, wrong and has been a running frustration for the PC ever since the development started but that's where we are.
FWIW, the clause about the trees is item 6 in
this document.
Eastnorthamptonshire District Council Planning dept can be contact on this number: 01832 742225 or emailed: planning@east-northamptonshire.gov.uk