In looking at the available data it seems like NR's asset database is not a reliable source for geodata. They have some grid refs but many of them are so clearly inaccurate (I'm not that au fait with Anglia region but I'm fairly sure Wisbech is on land, and not in the sea, as their data would suggest) that it's not worth bothering with them.
What I've done is take a network links model and used that to determine a reasonable location for each asset, since I have the start and end of each link.
The accuracy is... mostly ok, with about 90% being plus or minus a mile or so, and the worst being plus or minus 10 miles.
Not ideal, but workable, and something I can probably refine with some time and interpolation.