Are you saying that NR did less than they were paid by the "third party" to do?
The "third party" was the Welsh Government.
It's a long story but NR quoted a cost and WG stumped up that amount (£45 million from memory).
Having started, NR then said it would cost significantly more, and when told there was no more cash, stopped half way at Rossett and also failed to upgrade all the old track.
There were also repeated delays involving signalling at level crossings.
Throughout, NR didn't seem to care, and it left a sour taste locally and with WG.
Capacity has been marginally improved, but nothing like what double track throughout would have delivered (as at Loughor).
This was almost a decade ago now, when rail devolution wasn't a thing.
Now WG calls more shots over funding (and has taken over the Core Valley Lines), so NR now has to pay more attention.
Very few extra trains have run to use the new capacity, though reliability over the remaining single line is better.
New services to Liverpool will use the line, if they ever start (there's just one peak service each way now).