Front brake plate retaining nut - assistance please!

[Snowy]

Any advice appreciated!
Bike has had an upgrade with leading link front brake, but whatever I do I just can't stop the retaining nut coming loose and causing the most annoying rattle until braking takes up the slack!
Ok, I haven't the correct size spanner / socket, so tightening has been with stilsons / mole grips which is not the best method?
Any idea what the size of this nut is so at least I can have the correct spanner ready before taking it down!
Should there be a spacer between the nut and the fork leg when tightened up to stop nut loosening?
Will thread-lock be sufficient to hold it tight when tightened?
Should the fork legs be pulled tight together before tightening the axle clamps to remove any gap to the fork side of the nut?
As stated, any help appreciated, must get a workshop manual!

[JulianS]

I use an 11/16 whit box spanner. You would need quite a deep socket. I use a spare fork bottom fixed to the other end of the wheel spindle to hold it whilst tightening and also use thread locker.

If there is movement between brake plate and lug on fork leg the rocking tends to undo the nut.

Dont pull the fork bottoms together you will ruin the fork action

[BigJim]

I found an old large spanner that was slim enough to fit between the fork leg and brake plate. Then i filed it out to fit the nut nice and snug. This then used on rear wheel nut also
EDIT
Just saw you got the big twin leading shoe brakeplate. so doubt there room to get spanner in after fitting. Apologies
.

[Billybream]

Most important as Julian stated to ensure no play between brake plate and fork lug, to correct any play a simple piece of shim, bent over will help.