ISE AW23 Drive Belt

Once again I find myself shouting and swearing at my ISE AW23.
Following what can only be described as a shoddy repair by my local authorised agent 6 months ago it’s spat it’s drive belt off.
They appear to have reused the nyloc nut on the spindle and this then slowly comes undone causing a great deal of noise and finally popping the belt off.
They very reluctantly popped the old belt back on when this happened 6 months after they had me for £160 to repair a load of bits on it this time I’m doing it myself…..how hard can it be?
Answer…..bloody impossible!
There must be a trick to it, I’ve bought a new belt, but there’s no way it’ll stretch far enough, if only the back came off instead of the top. 😕
And what is the thread on the spindle?
The butchers/ISE agent have made a right mess of the threads and neither a standard M12 or 1/2″ UNC nut go on ‘smoothly’ so unless I can find the right thread size the old battered thread nut is going to have to back on but with a shakeproof washer on.

Currently loosing the battle, and the skin on my knuckles but if I have to cut the back off the bloody thing isn’t beating me! 😆