Reputable Gearbox Recon or Repair

[BlackG7R]

Sounds like the gearbox on my nephews Vauxhall Corsa 1.2 maybe kaput.

Can anyone recommend a reputable and reliable company to diagnose / repair / or swap out for a recon box.

Obviously trying to avoid the backstreet "give it a pressure wash, a lick of paint, and sell it on as reconditioned" merchants.

Thanks in advance.