Thanks for all the help everyone. So just to summarise and check my work so I am not missing anything:
1) Strandberg is brand new and appears to be well grounded at all points when checking with a multimeter.
2) Noisy in humbucker positions and quieter in split coil positions. Otherwise the tone is loud, clear and glorious.
3) Tried different cables to no effect.
4) Touching strings/output jack or not makes no difference to hum.
5) Visual inspection of solder joints looks good and confirmed wiring is correct as per Strandberg wiring diagram.
6) I have another guitar which, in neck position only, buzzes similarly in same playing position. Other humbucking guitars seem quiet though in same circumstance.
7) I am an idiot it seems and have been sitting in front of a lot of EMI noisy equipment and never thought about it till now, including 2 x NAS full of drives, big monitor, rack with gear, power supplies etc on desk right in front of me facepalm
8) If I move around the room I can find locations where the problem disappears completely and guitar is quiet even with massive amounts of gain. Having pickups parallel with the ceiling is actually the best but not great for playing!!
9) There is a breaker box not too far from where I sit. Holding the guitar close to that gets some extreme buzzing happening!!
So from all this, is it most likely an environmental problem and not an issue with the Strandberg wiring or pickups? Perhaps some of my other guitars just reject noise better (but not all given the noisy neck position described of one of the other guitars)?
When I get some time next I will turn off all the equipment on the desk and test again. Will then turn on gear one by one to see if I can isolate the major cause or if there is a heap of causes environmentally lol.
Strange I haven’t noticed any of this before though having other guitars.
Does this sound about right?