I'm tempted to say Llandaff / Canterbury - because of the Monmouthshire geography, whereas on the Gloucestershire side of the Wye I would have said Gloucester or London / Canterbury.

The Prerogative Court "sat" in various places commonly Canterbury or York.
If your ancestor lived south of the River Humber or in Wales and was relatively wealthy or died after 1800 there is a fair chance that the will was proved in the Prerogative Court of Canterbury (PCC)

If your ancestor was relatively wealthy and lived in the northern counties then it is possible that the will was proved in the central Prerogative Court of York (PYC)

Otherwise if none of the above applies, it is most likely that the will was proved in one of the many lower level ecclesiastical courts of which there were over 250

The records for Wales are held at the National Library of Wales.

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Ἀριστοτέλης A Gloster Boy in the Forest of Dean ><((((*>