Navaids
Many thanks for your comments and information. I had assumed that GPS was now king and the standard longhaul navaid. From your comments, it would seem that the standard is INS/IRS updated by GPS (and/or DME)
However, it would also seem that both INS/IRS and twin GPSS are sufficient on their own. From your further comments, it would appear that aircraft may legally fly long-haul over water equipped with either of the two but ideally with both, one updating the other.
Thank you again for the information