Right, after looking around and doing some digging I figured this....
Shimano Aero Specimen 18lb.
It seem Shimano manufacture about +30% tolerance into their line - based on that chart it seems consistent. I hope that it would be through their range, but that is where that chart puts the curve ball in.
It also appears that they are reasonably consistent with their advertised line diameters.
Theoretically, then, if you would be prepared to accept these positives and ignore the potential that their Aero line could fall outside the above tolerances (certainly possible) then the 18 should run between 23-26lb average at 0.36mm?! Sounds reasonably good to me. If you factor cost into it too then even better.
So, forget the second spools and just load up with this for the season I think. My only concern is how well it breaks surface tension to start sinking and how supple it is. Any guesses?
And yeah, brilliant link mate, properly helped me out!
Otherwise, it'll be back on to Daiwa Sensor and we'll deal with some springing!