Hi Matty, you might get more success starting a new thread...
However, personally, I am not an advocate of having a nut each side the rim, I have tried this and find the tube isn't sitting snugly to the rim, the other thing I do is NOT clamp the nuts down from the top of the rim either, Instead I screw them up against the dust cap. These nuts will never hold the tube in place and if the tube does creep then before you know it you will have ripped the valve out with no sign of anything (if you have a nut each side you have even less warning as the valve will still sit pert on the rim). with no nuts locking you'll see the valve angled one way or the other giving plenty of time to let out some air and move it along the rim.
as for why its leaking air, its possible that there is air trapped between the tube and tyre and slowly working its way out (if you've had the tyre off recently) if youve inflated the tube and submerged the tube alone in water then there shouldn't be a problem there... Either that or the valve could be leaking itself and with not being mounted isn't adding enough external pressure to allow any to leak out. a change of valve core sometimes works or a bit of spit on the end of the valve then pump it up often works too.
Now let’s all have a beer
68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)